Posted by steve on September 11, 2002 at 3:04 PM:
January (Ichi-gatsu)
1/1 Ganjitsu (Ganjitsu)
1/3 First Beginning Ceremony (Genshi-Sai)
1/7 Seven Herbs Festival (Nanakusa)
1/14 Snow Festival (Yuki Matsuri)
February (Ni- Gatsu)
2/1 Bean Throwing Ceremony (Setsuban)
2/2 Rice Ceremony (Gohanshiki)
2/3 Lantern Lighting Ceremony (Mando-e)
2/4 Demon Driving Ceremony (Onioi-shiki)
2/12 First Horse Day (Hatusu-uma Matsuri)
March (San-Gatsu)
3/3 Girl's Day/Peach Blossom Festival (Momo-no-Sekku)
3/20 spring equinox (Shumbun-no-hi)
3/23 Spring Imperial Spirit Festival (Shunki-Korei-San)
April (Shi-Gatsu)
4/8 Buddha's Birthday (Hana Matsuri)
4/29 Emperor's Birthday (Tenno Tanjobi)
May (Go-Gatsu)
5/5 Children's Day (Kodomo-no-hi)
Boy's Day (Tango-no-sekku)
Night Festival (Fuchu Kurayami Matsuri)
5/12 Boat Festival (Mifune Matsuri)
5/15 Hollyhock Festival (Aoi Matsuri)
June (Roku-Gatsu)
6/21 summer solstice (Geshi)
July (Shichi-gatsu)
7/7 Weaving Loom Festival (Tanabata Matsuri)
7/13 - 7/15 Feast of Lanterns (O-bon)
7/14 Fire Festival (Nachi hi Matsuri)
Rice Planting Ceremony (Sumiyoshi Otaue Shinji)
7/15 Light Offering Event (Ishizaki Hotosa)
7/16 Music Festival (Kangen-sai Matsuri)
7/17 Gion Matsuri
7/20 Festival of the Canals (Suitosa)
7/23 Matsun-o Matsuri
7/24 Tenjin Matsuri
7/25 Lantern Offering (Yahiko Toroshinji)
August (Hachi-Gatsu)
September (Ku-Gatsu)
9/23 autumn equinox
October (Ju-Gatsu)
November (Juichi-Gatsu)
December (Juni-Gatsu)
12/21 winter solstice (Toji)
I am not sure how up-to-date this list is, the books that I found these in were not very recent... a lot of festivals are held in differant regions and vary on the dates, others are held all across Japan. There are probably plenty more in the later months, I never finished making notes of their dates... Also, I've no idea which ones might be celebrated out-side of Japan..