These are the chief holidays and the dates on which they take place:

Estuary 1 Entharion Day
Frobuary 3 Undergroundhog's Day
Frobuary 29 Start of Antharia Leap Week (Antharia only)
Arch 11 St. Balhu's Day
Oracle 3 King Wurb's Birthday
Oracle 4 St. Foobus' Day
Oracle 22 Coronation Day
Mage 0 Zero Day
Mage 1 Mage Day
Mage 31 St. Honko's Day
Jelly 4 Filfre Day (The Zork of Jelly)
Jelly 25 St. Quakko's Day (maybe)
Augur 6 St. Bovus' Day
1st Mud Day in Suspendur Leisure Day
Ottobur 6 St. Wiskus' Day
Ottobur 31Halloween
Mumberbur 11 Veterinarian's Day
Mumberbur 14 Curse Day
Dismembur 25 Christmas (ancient holiday)

SOURCE(S): Zork Zero, Legends of Zork