Lutherans observe St. Michael and All Angels in our liturgical calendar because it is a feast day that points directly to Christ. Throughout Scripture angels are warriors and messengers, appearing on God’s command to deliver either judgment or His Gospel message. They announced Jesus’ birth, ministered to Him after Satan tempted him in the wilderness, and were present at His resurrection and ascension. We celebrate Michaelmas (an old name for the day) because it points to Jesus.
Philipp Melanchthon, a colleague of Martin Luther, wrote a hymn commemorating the day. You can find “Lord God, to Thee We Give All Praise” in the Lutheran Service Book, number 522.
Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back, but he was defeated, and there was no longer any place for them in heaven. And 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. And I heard a loud voice in heaven, saying, “Now the salvation and the power and the kingdom of our God and the authority of his Christ have come, for the accuser of our brothers has been thrown down, who accuses them day and night before our God. And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death. Therefore, rejoice, O heavens and you who dwell in them! But woe to you, O earth and sea, for the devil has come down to you in great wrath, because he knows that his time is short!”