The AHA style guidelines. The BJCP style guidelines.

Jan: English Brown Ale (BJCP Cat 11)
Feb: Bock (BJCP Cat 5)
Mar: American Ale (BJCP Cat 10)
Apr: Belgian Strong Ale (BJCP Cat 18)
May: Specialty Beer (BCJP Cat 23 & Cat 16E)
Jun: Amber Hybrid Beer (BJCP Cat 7)
Jul: IPA (BJCP Cat 14)
Aug: Scottish and Irish Ale (BCJP Cat 9)
Sep: Stout (BCJP Cat 13)
Oct: Oktoberfest (BJCP Cat 3B)
Nov: Strong Ale (BJCP Cat 19)
Dec: Fruit Beer (BJCP Cat. 20)