Michaelmas Day is also known as the Feast of St. Michael and All Angels. It is a holy day, and is celebrated on the 29th of September. While angels are mentioned throughout the Bible, only four of them are mentioned by name - Michael, Gabriel, Raphael and Uriel. The word 'angel' comes from the Greek word for 'messenger' - they are the messengers of God to humankind.
Saint Michael is often portrayed in art as a warrior angel - fully armed with a helmet, sword, and a shield which often bears the inscription "Quis ut Deus," or "Who is like God," which is the meaning of his Hebrew name. Traditionally Saint Michael has four offices: to fight Satan; rescue the souls of the faithful at the hour of death; to be the champion of God's people; and to call human souls to judgment.
According to Christian tradition, before creation, there was a war in heaven, and Michael the archangel and the angels of light defeated Lucifer and the fallen angels. There is a similar account of a battle in heaven recorded in the Book of Revelation. "And war broke out in heaven; Michael and his angels fought against the dragon. The dragon and his angels fought back, but they were defeated, and there was no longer any place for them in heaven. The great dragon was thrown down, that ancient serpent, who is called the Devil and Satan, the deceiver of the whole world – he was thrown down to the earth, and his angels were thrown down with him." (Revelation 12.7-9)
This description of angels is a far cry from the baby faced cherubim we know from renaissance art. Michaelmas was traditionally one of the quarter days when accounts were settled. Traditional meals included goose, and a special cake called a Saint Michael's bannock. The Feast of Saint Michael and All Angels is celebrated near the autumn equinox and is associated with the beginning of autumn, and many universities name the first term Michaelmas.
This is the last vestige from a time when Michaelmas was celebrated in the church as a season, ending, originally on the feast of St. Martin of Tours (November 11th) when Advent was celebrated as a pre-Christmas Lent.