On this day the Church recalls with joy the ministry of angels, those special messengers of God who, Scripture records, sometimes visit humans with messages from God. Perhaps the most important messenger was that of the Archangel Gabriel, who announced to the Virgin Mary the news that she would bear a Son. Given that, I don't know why this feast isn't "St. Gabriel and All Angels," but there it is.
Another name for this day is Michaelmas, and it's the name for the fall term at General Theological Seminary and at Oxford and Cambridge, among others, as well. Incoming students matriculate at General on Michaelmas. At Evensong on this day, each new student signs the Matriculation Book, which, in a single volume, contains the signatures of every student who has ever attended GTS since its founding in 1819. Let me tell you, it was with both joy and awe that I myself signed The Book on this day in 2003.
Everlasting God, you have ordained and constituted in a wonderful order the ministries of angels and mortals: Mercifully grant that, as your holy angels always serve and worship you in heaven, so by your appointment they may help and defend us here on earth; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever.