Michaelmas the feast of St Michael and All Angels, 29 September, one of the quarter days in England, Ireland, and Wales. Old Michaelmas is the day that would have been called 29 September if the Old Style calendar had not been corrected.
Michaelmas goose is traditionally eaten at a feast on this day.
Michaelmas summer a period of warm weather before the onset of winter, associated with this time.

Michaelmas feast of St. Michael the archangel, 29 September (a quarter-day). OE. sancte Micheles mæsse Saint Michael's mass (see MASS1), ME. Mi(ʒh)elmasse (XIII), Mykylmes (XV).

Michaelmas Christian feast day of St Michael and All Angels, celebrated on September 29. Since the Middle Ages, Michaelmas has been one of the four quarter days of the financial year.