In the Touhou Project fandom, there are several days celebrated by fans, usually by posting fanart. This page forms a (hopefully) comprehensive list of days in use, presented in English for the Touhou Project's sizable western audience.
The majority of the days are goroawase, i.e. numeric puns in Japanese. Several others are derived from birth flowers associated with certain days. Details of these can be harder to research, as there are several different birthday flower lists in use.
The majority of the day definitions are derived from entries on the Pixiv Encyclopedia page for Touhou Character Days. An alternative list can be found at Touhouwiki.cc.
Any missing days, corrections, or additional notes can be suggested via DM to @TouhouCalendar on Twitter. The dataset in machine-readable YAML format is available on GitHub.
Goroawase: 1(I)4(shi)=Ishi (Stone)
Goroawase: 1(Ichi)5(go)=Ichigo (Strawberry)
Goroawase: 0(Re)1(i)6(mu)
Goroawase: 1(Hi)7(na)
Goroawase: 1(I)8(ba)=Ibaraki
Goroawase: 1(I)9(Ku)
Goroawase: 0(Rei)1(i)10(den)=Touhou Reiiden~ The Highly Responsive to Prayers
Goroawase: "One" sounds like "wan", onomatopeia for a dog barking. "0" read as "Oh", like a wolf's howl.
Goroawase: 1(i)10(to)=糸 (Thread), alluding to threads of spider silk.
Goroawase: 1(i)10(do)=井戸 (Well). Kisume is of course associated with wells, as she lives in a well bucket.
1/16 is 閻魔賽日, a buddhist holiday that relates to Yama.
1/22 is Jazz Day in Japan, and trumpets are prominent in jazz.
Anniversary of the release date of "音盤 はたらきもの", inspiration for Ellen.
Goroawase: 1(Hi)2(fu)9(Ku)=Hifuu Club, and the birth flower of 1/29 is the sumire.
Goroawase: 2(Fu)2(Two)=普通(Futsu) (Ordinary i.e. Ordinary Magician)
Goroawase: 2(Ni)2(Ni)=Nyan Nyan/Niang Niang, part of Seiga's nickname title.
Goroawase: 2(Ni)4(shi)=Niwashi (Gardener), like Youmu
Goroawase: 2(Fu)4(ji)
Goroawase: 2(Fu)0(maru)6(roku)=Touhou Fuumaroku~ the Story of Eastern Wonderland
Historical day: 2/7 was the real Prince Shōtoku's birthday.
Goroawase: 2(Ni)8(wa)=Niwatori (chicken)
Goroawase: 2(Ji)9(ku)=Touhou Yumejikuu~ the Phantasmagoria of Dim.Dream
Goroawase: 2(Fu)10(to)
Goroawase: 2(Ni)10(to)
Goroawase: 2(Ni/Fu)10(to)
Goroawase: 2(Futatsu)1(i)0(wa)
Goroawase: 2(Ni)10(to)
Goroawase: 2(Futatsu)1(i)8(wa)
Goroawase: 2(Fu)2(ji)3(mi)=Fujimi. The PCB character profiles refer to Yuyuko as "富士見の娘" ("Fujimi's Daughter"). The Saigyou Ayakashi and Yuyuko's surname Saigyouji are named after the poet Saigyo, best known for poetry and images about looking at Mount Fuji (fujimi).
Goroawase: 2(Fu)と(To)2(ji)5(ko)
Goroawase: 2(Fu)2(ni)9(ku)=腐肉 (carrion)
Goroawase: 3(Sa)2(ni)=Sunny
Hinamatsuri, obviously.
Goroawase: 3(Mi)4(yon)
Goroawase: 3(Mi)5(ko)
Goroawase: 3(Mi)5(ko)
Goroawase: 3(san->zan)6(mu)
Goroawase: 3(Sa)7(Na)
Goroawase: 3(Mi)7(na)(mitsu)
Goroawase: 3(Mi)9(Gu)=Mi(sha)Gu(ji)
Goroawase: 3(Sa)9(ku)
Goroawase: 3(Sa)9(gu)
Goroawase: 3(Sa)10(Tori)
Goroawase: 3(Sa)10(Ten->Tenshi (Angel)->Riel)?
Goroawase: 3(Mi)10(to)2(ji)
Goroawase: 1(I)7(na)8(ba)
Goroawase: 3(Mi)20(Futo)
Goroawase: 3(Sa)2(ni)1(i)=Sunny
Goroawase: 3(San)2(ni)4(yo)=Sannyo
Goroawase: 4(Yo)6(mu)
Goroawase: 4(Shi)10(To)=しっと (jealousy)
4/23 is Book Day, and Kosuzu is associated with books
Goroawase: 4(Shi)2(tsu)8(ha)
Goroawase: 4(Shi)3(sa)0(wa)=Shi(ra)sawa
In France, May/Labour Day is Fête du Muguet, as the lily of the valley (Muguet in French) symbolizes the return of happiness. Medicine is associated with the lily of the valley as well. The tag name, "Konparo Festival", references a line Medicine says in PoFV.
Mystic Square was released at C55.
Goroawase: 5(Ko)5(ko)6(ro)
Etymology unknown. The tag name is a portmanteau of "手乗り" (Tenori, To hold/cradle in the hand) and "りぐる" (Wriggle)
Pun: May sounds like '目' (eye), making this "5 Eyes".
Goroawase: 5(Ko)9(Ku)=Ko(a)Ku(ma)
Goroawase: 5(Ko)10(to)
Goroawase: 5(Ko)1(i)4(shi)
Odd pun: May + 15th periodic element is P (rin in JP, same as in Orin's name)
Goroawase: 5(Ko)1(i)5(ko)
If the first 5 is changed to the similar-looking kana 'ら', it reads as ら(ra)1(i)5(go/ko)=Raiko
Goroawase: 5(Ko)1(i)6(ro)=Koiiro (Love-colored)
Goroawase: 5(Ko)1(i)8(ya)=Kochiya
The JP birthstone for 5/19 is lapis lazuli (rough)
Goroawase: 5(ko)x2=koko 6(ro). Also, release date of HM
Goroawase: 6(Mu) 1(Ichi)
Mai's theme is "裏切りの少女 ～ Judas Kiss" meaning "Treacherous Maiden ～ Judas Kiss". 6/2 is known as Treachery/Betrayal Day (裏切りの日) as it is the anniversary of Oda Nobunaga being betrayed by Akechi Mitsuhide.
Goroawase: 6(Mu(rasa)) 3(Mi(namitsu))
Goroawase: 6(Mu)4(Shi)=虫 (insect)
Pun: June + 5(Go/Ko) = Junko
Goroawase: 6(Roku)6(roku)=rokurokubi, Sekibanki's species.
Goroawase: 6(ro)6(ro)-> Kerokero, frog croaking onomatopoeia. 6/6 is therefore JP Frog day, and Suwako is associated with frogs.
"6月7日" has 6 (Junko, 6th boss) and 7 (Hecatia, 7th boss) around the kanji meaning month/moon (月).
Goroawase: 6(Mu)9(Kyu)=Mukyuu, Patchouli's "catchphrase".
6/9 looks like the ying-yang symbol, one of Shingyoku's forms.
The day after 6/9, Patchouli day.
Umbrella day was established by the Japan Umbrella Promotion Council.
Goroawase: 6(Mu)1(i)4(shi) = Muishiki (無意識), meaning "the unconscious" in psychology, a theme for Koishi.
Midpoint between love-colored day and seven-colored day
6/18 is Hotate (oyster) day, and Hatate is one letter off from Hotate.
6/24 is World UFO Day.
6/27 is Lafcadio Hearn/Koizumi Yakumo's birthday
Goroawase: Goroawase: 7(na)2(tsu->zu)=Nazrin
The "95th" day of April, referencing Flandre's spellcard "Ripples of 495 Years"
Goroawase: 7(Nana)4(shi)="no name"
Goroawase: 7(Na)6(mu)=Namu(san)
7/6 is the anniversary of the introduction of the piano to Japan.
Related to her catchphrase "So Nanoka" and 7(Na)
7/7 is Tanabata (Star Festival), and Marisa is star-themed.
Flan Day (4/95 or 7/4), plus 5 days to represent the supposed 5 year gap in their ages.
July is also spelled 文月 and Cirno is associated with 9.
193rd day of the year. Goroawase 1(I)9(ku)3(san)
Sunflower Day was established in 1977 in celebration of the launch of the Himawari (sunflower) geostationary meteorological satellite over Japan. Of course, Yuuka is associated with sunflowers.
Sunflower day is in July, which can also be written as 文月 (bungetsu), wth the same character in Aya's name.
Goroawase: 7(Nana)1(i)6(ro)=Nanairo (seven-colored)
Goroawase: 7(Na)2(zu)0(rin)
One tweet says it's because the Japanese sword "Yamanbagiri Kunihiro" was designated as an important cultural property on June 21st, 1962. As the name indicates, its legend is directly associated with Yamanba. Another says that the birth flower of June 21st is the "Nemunoki", which has a clear name connection to Nemuno.
July can also be written as 文月 (bungetsu), and 2(fu)3(mi) is an alternate reading for the kanji 文.
Kana Anaberal is a poltergeist. 7/26 is Yuurei (ghost) day.
Suika's name is a homophone for the Japanese word for watermelon. The day 7/27 was dubbed watermelon day because the stripes of a watermelon resemble a rope, and 7/27 is read similarly to the phrase "夏の綱" or "Summer Rope".
Konngara is based off of the 7th of the 8 great youths of Fudoo Myoo-oo. The 28th of every month is the day to celebrate Fudoo Myoo-oo.
8/1 is Water Day (水の日)
8/1 is Water Day (水の日)
Goroawase: 8(Ha)2(ni)
Goroawase: 8(Ba)2(ni)=Bunny
Goroawase: 8(Ya)3(sa)日(ka)
Goroawase: 8(Ya)/3(mi)=Yami, from Meira's theme Yami no Chikara ("やみのちから")
Goroawase: 8(Ya)/4(yo)=Yatteyanyo! (やってやんよ!), Tojiko's "catchphrase" and from TD.
Goroawase: 8(Ha)4(shi)=Hashi(hime)
Goroawase: 8(Ya)4(Hoo)!
Goroawase: 85頃=Yagokoro=”almost 85”
Goroawase: 8(Ha)5(Ko)=Ha(tano) Ko(koro)
Goroawase: 8(You)6(mu). Reading 8 as "You" seems like a bit of a stretch...
Goroawase: 08(Aya)/06(Reimu)
Goroawase: 8(Ya)7(ma)
Goroawase: 8(Ha)7(na)=Hana (Flower)
Keyboard day, as a typical piano keyboard has 88 keys.
8/8 is the midpoint between the days of fellow "Team 9" members Rumia (7/7) and Cirno (9/9).
8/8 can be flipped or rotated, and still read 8/8, making it perfect for a Seija day.
8*8 = 6(Mu)4(shi) (Bug)
8/8 is Butterfly day, as the numbers resemble a butterfly.
Goroawase: 8(Ya)8(Ha). Her "sister" is also regularly included.
Goroawase: 8(Ya)9(ku)=Yaku(jin), curse god, Hina's species
Goroawase: 8(Ha)9(ku)
Goroawase: 8(Ba)9(ku)
Goroawase: 8(Ya)9(ku)
Goroawase: 8(Ba)9(gu)=bug
Either goroawase: 8(Ha)10(te(n))=Hatate, or a pun off of Oyster (Hotate) Day
8/11 is Mountain Day (山の日), and Takane is a Yamawaro (lit. "mountain child").
Goroawase: 6(Ru)7(na)3(sa)
Goroawase: 8(ya)20(nitou)
8(Ha)2(ni)0(wa)
"Perfect Day" was established on August 21, 1970 by Motherland Co. after Ritsuko Nakayama became the first female professional bowler to achieve a perfect score in a professional championship match. By association with Cirno's "Perfect Freeze" spellcard.
Sum of Satori Day (3/10) and Koishi Day (5/14).
Goroawase: 8(Ha)2(ni)8(wa)
Goroawase: 8(Ha)2(ni)8(wa)/8(Ha)2(ni)8(ya)
Goroawase: 9(Ke)6(ro)=kero (frog onomatopoeia)
Goroawase: 0(A?)8(ya) 3(Sa)7(na)
Goroawase: 9(Ku)6(ro)=Kuro (black), i.e. Kurokoma, Prince Shōtoku's Black Pegasus.
Goroawase: 9(Ku)6(ro)=Kuro (black), tying into Eiki's black-and-white judgements.
Goroawase: 9(Gu)8(ya)
You already know why.
Goroawase: 9(ku(moi)) 1(ichi)0(rin)
The number 13 resembles the letter B, which is a homonym for 尾 (tail), making this 9尾 or "9 Tails." Ran is a nine-tailed kitsune.
Per the linked Pixiv Dictionary post, 付喪神→つくも→九十九→919→9月19日
This is tsukumogami sisters day, and Kogasa is a sort of tsukumogami
The Aki sisters are associated with autumn, as their surname Aki literally means autumn.
Approximately the Autumn Equinox, around which a notable full moon called the Harvest Moon occurs - fitting for a werewolf
Kurumi means "walnut", and so this pigybacks off of Walnut Day. The date derives from 9(Ku)3(mi) being like "Kurumi", and additionally 0 resembling a round walnut on a tree.
Anniversary of the publication of Touhou Kourindou
Goroawase: 10(Do)0(rei)3(mi)
Goroawase: "Teruyo" is like 10(Ten/To)4(yo)
Goroawase: 10(Ten)4(shi)
Goroawase: 10(to)4(yo)
Goroawase: 10(Ten)5(ko)
Goroawase: 10(Ten)6(Mu)=Ten Musume (Heaven Girl)
The Japanese character for 10 (十) looks like Rumia's iconic t-pose (as discussed in her EoSD dialogue). The kanji for 10 can also be read as "so", in combination with 7(na), it forms the first characters of her "catchphrase" "So nano ka?"
Goroawase: 10(Ten)9(Gu)
Goroawase: 10(Ten)9(Kyu)
10 in hexidecimal is A, and 9 can be read as "Kyu", hence Akyuu.
Goroawase: 1(Ichi)0(rin). Doubled, so it's super.
Ten sounds like Chen!
10/10, like Ten Desires.
Oct. 15 is both National Mushroom Day and a Japanese Doll Day. Marisa is associated with mushrooms, and Alice is associated with dolls.
Goroawase: 10 1(i)6(ro)=10 Colored. The idea is that Alice's seven colors, Reimu's red and white, and Marisa's black and white sum to ten colors (if you consider white is common between them, sort of).
Goroawase: 10(To)2(ji)5(ko)
10/26 is Atomic Power Day in Japan.
Gorowase: 1028 = 2*514, and 5(Ko)1(i)4(shi)
Goroawase: 1(i)1(i)0(Re)5(ko)=ii Renko (Good Renko)
Goroawase: 1(i)1(i)0(Rei)6(mu)=ii Reimu (Good Reimu)
Goroawase: 1(i)1(i)0(O)9(kyuu)=ii Okuu (Good Okuu)
Goroawase: 1(i)1(i)10(do)=いい井戸 (Good Well)
In Strange Creators of Outside World vol. 5's cross review, Satori rates herself at an 11/10 due to how powerful (she thinks) mind reading is. As such, she is memetically linked to the number 11.
11/11 looks like onbashira sticking up from the ground.
Goroawase: 11(ii, JP for "good") 12 (hifuu, as in hifuu club)
11/14 was dubbed Good Stone Day (11(ii)14(ishi)=いい石) by Yamanashi Stone Industry Association in 1999. Eika likes to stack stones.
The Birth Flower on 11/18 is the Himejo-en (姫女苑), with the same characters as Jo'on's name.
Goroawase: 11(ii)1(i)9(ku) = Ii Iku, "Good Iku"
Goroawase: 11(ii)24(nishi)=Ii Niwashi (Good Gardener), like Youmu.
Goroawase: 1(i)1(i)2(zu)7(na)=Iizuna(maru)
December's birth stone is Lapis Lazuli (at least, in the National Association of Jewelers' original 1912 list). The 04 is a goroawase pun 0(wa)4(yon)=wayon, a phrase spoken by Hecatia in the LoLK Extra Stage. It sometimes serves as a fanon catchphrase of hers, similar to "mukyuu" for Patchouli and "myon" for Youmu.
Goroawase: 1(Hi)2(Fu) 9(Ku) = Hifuu Club.
Goroawase: 1(Ichi)2(Fu)10(To)
Goroawase: 1(Hi)2(fu) 1(Hi)2(fu) = Hifuu (secret), as in Hifuu Club.
Letty, as a yuki-onna, is associated with winter.
Portmanteau of Yakujin (curse god, Hina's species) and Christmas.