The feast of the archangel Michael, a church festival, celebrated on the 29th of September. Hence, colloquially, autumn.
Feast of St. Michael on 29 Sept. (MEDIEV-L. Medieval Terms)
Term October-December - Michaelmas
the Feast of St Michael (29 Sept)
honoring the archangel Michael; a quarter day in England, Wales, and Ireland
The 29th of September, feast day of St. Michael and the beginning of autumn
29 September, St Michael's day and one of the 4 'quarter days' when leases took effect
A term in the legal calendar (usually from 6 October-25 November) or 29th September (as a date)
the Autumn term
Michaelmas (pronounced ), or the Feast of Ss. Michael, Gabriel and Raphael, is a day in the Christian calendar, taking place on 29 September. Because it falls near the equinox, it is associated with the beginning of Autumn and the shortening of days.