Setup a circle a set of chairs with one less chair then the amount of people present.
Make one person "it".
Go around the circle and have each person state which month he or she is born in. The "it" person calls a month and everyone with that month must switch seats. The "it" can also call multiple months and if the "it" would like everyone to move seats the person calls out "Birthday Bash" and then everyone is required to switch seats. The person left out is the new "it".