Cosmos Carnival (Late February - Early March, week long) - The Feast of Fools. Mardi Gras. A long party week. All classes throwing caution to the wind and dressing in as elaborate of masques and costumes as they can afford. Even the Church takes a more lax view on sin during this single week. And the League loves it, because they sell a lot more booze than usual. Finally, of interest to Yathrib, the Kurgans also celebrate this holiday, as the birthday of their own strange prophet.
Urth Day (May 1st) - A holiday dedicated to Mankind's Homeworld, Holy Terra.
First Jump (July 4th) - The celebration of the first leap into the Stars by Mankind. Mainly celebrated by Charioteers and Spacers.
Saint Ramos Day/Colony Day (August 10th) - A Holiday Celebrating a pre-reflective Saint adopted by the Church for his struggles against the Zaibatsu. The League sees it in a different light, as a worker's holiday that Ramos was killed opposing.
Santa Flama Dies (Sep 17) - A Day of Prayer and Contemplation. Also of plays and magic lantern shows about the Prophet and his Disciplies throughout the Known Worlds.
Mortos Diablos (Oct 31) - Halloween, it's survived.
Dies Lamentum (Dec 23) - the day of the Prophet's Death. A very Holy Day, religious in nature. Expect to go to Church! The most devout self mortificate.
Lux Splendor (Dec 25) - The most important holiday in the Known Worlds, celebrating the Prophet's radiant light cleansing the Jumpgates after his death. Gifts are commonly given.