Definitions for michaelmasˈmɪk əl məs
Random House Webster's College Dictionary
Mich•ael•masˈmɪk əl məs(n.)
Sept. 29, celebrated as the feast of the archangel Michael.
Origin of Michaelmas:
bef. 1150; ME Mighelmes; OE (Sanct) Michaeles masse (St.) Michael's Mass
Michaelmas, Michaelmas Day, September 29(noun)
honoring the archangel Michael; a quarter day in England, Wales, and Ireland
A Christian feast in honour of the archangel Michael
29th September, the day it is celebrated
the feat of the archangel Michael, a church festival, celebrated on the 29th of September. Hence, colloquially, autumn
The Nuttall Encyclopedia
is the festival in honour of St. Michael and the angels, held on the 20th September, the day being one of the quarter days on which rents are levied.