The Feast of St. Micheal, or Michaelmas occurs on September 29 and falls near the equinox and is associated with the beginning of Autumn and the shortening of days. In medieval times  four "quarter days" were important, namely: Christmas, Lady Day, Midsummer Day and Michaelmas.  While these days were the basis for financial calculations, they are also loosely coincide with the summer and winter solstices, and the spring and autumnal equinox.

The 29th of September marks the end of the harvest and this feast is associated with the Archangel Michael, one of the principal angelic warriors.  Michael is seen as a protector against the dark of night, and the administrator of cosmic intelligence.

In the Anglican and Episcopal traditions there are three or four named Archangels:  Michael, Gabriael, Raphael, and Urial. There are three archangels mentioned in scripture, the other archangels are from the Book of Enoch--Uriel, Raguel, Sariel, and Jeramiel.  Michael's name in Hebrew means "who is like God."

St. Michael is usually  portrayed as a warlike figure, and is  referenced in the Golden Legend, written in 1275 by Jacobus de Voragine, Archbishop of Genoa.  "The fourth victory is that the archangel Michael shall have of Antichrist when he shall slay him. Then Michael, the great prince, shall arise...Michael shall come and shall slay him."

Michael is considered the patron of knights, policemen, soldiers, and other warriors.  He is regarded in the Christian world as the chief of angels, or archangel. His history is obscure. In Scripture, he is mentioned five times; namely, thrice by Daniel as fighting for the Jewish church against Persia; once by St. Jude as fighting With the devil about the body of Moses; and once by St. John as fighting at the head of his angelic troops against the dragon and his host.  Sometimes Michael is represented as the sole archangel, sometimes as only the head of a fraternity of archangels. 

The full moon nearest to Michaelmas (strictly speaking nearest the autumnal equinox) has special properties, and is known as the harvest or hunter's moon.  In most months the interval between sunset the full moon appearing is about 45 minutes, but around the spring and autumn equinox this interval is reduced to only 25 minutes.  Thus a harvest full moon can artificially extend the evening light, which is useful for farmers and for those hunting wildlife. 

Michaelmas Day is about a week after the Vernal Equinox.