St. Michael is the greatest of all the archangels and is honoured for his defeat of Lucifer. He is one of the principal angelic warriors, seen as a protector against the dark of night, and the administrator of cosmic intelligence. Because Michaelmas falls near equinox it is associated in the northern hemisphere with the beginning of autumn and the shortening of days. Michaelmas is still celebrated in the Waldorf schools....a special happy Michaelmas festival to you :O)
The Archangel Michael, from the Altarpiece of the Last Judgement by Rogier van der Weyden link
The Archangel Michael, 1480-95 by Juan de la Abadía, known as 'The Elder', active in Huesca, 1469-1498 link
In England, the entire autumn season is known traditionally as Michaelmas. Many fairs took place during the harvest time, and since the Middle Ages Michaelmas fairs were focal gathering for farmers and merchants. Big goose fairs were held several places (fx. Nottingham Goose Fair) in UK...and the traditional meal for the Michaelmas Day includes goose and a special cake called a St Michael's bannock (take a look here)
Old Michaelmas Day falls on October 11th (October 10th according to some sources). According to an old legend, blackberries should not be picked after this date. This is because, so folklore goes, Satan was banished from Heaven, on this day, fell into a blackberry bush and cursed the brambles as he fell into them.