The Feast of St. Michael the Archangel occurs on September 29. 

A novena is a prayer prayed on nine consecutive days with a specific intention. It may be made at any time of the year, but a popular time to pray a Novena to St. Michael the Archangel is on each of the nine days from Sept. 20 to 28.

There are various ways to pray a Novena to St. Michael. One common method is simply to pray the Prayer to St. Michael on each day:

Prayer to St. Michael
St. Michael the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil. May God rebuke him, we humbly pray; and do thou, O prince of the heavenly host, by the power of God, cast into Hell Satan and all the evil spirits who prowl about the world seeking the ruin of souls. Amen.

Alternatively, here are two other popular guides for praying a Novena to St. Michael, available at the following links: