Webster’s Revised Unabridged Dictionary · 1913
Michaelmas
Mich′ael·mas (mĭk′ĕl·mas), n. [Michael + mass religious service; OE. Mighelmesse.] The feast of the archangel Michael, a church festival, celebrated on the 29th of September. Hence, colloquially, autumn.
Michaelmas daisy. (Bot.) See under Daisy.