The feast that defines the season, since so many terms are called ‘Michaelmas’.  Also, one of the quarter days.  A time to settle debts. 

Flowers: Michaelmas daisy; Our Michaelmas daisies (I planted one plant last autumn) have been out for ages already, and really looked fantastic.
Cake: Angel cake
Food: goose (traditionally, since they used to be in season about now).
Prayer.  There is a prayer to St Michael, but I forget how it goes.  I think there’s a hymn too, probably similarly forgotten.