THE NATIVITY FAST begins this Thursday, November 15th, and continues through December 24th. This Fast is divided into two periods. The first period is November 15th through December 19th when the traditional fasting discipline (no meat, poultry, eggs, dairy, fish, wine and oil) is observed on Monday, Wednesday and Friday, but with katalysis [allowance] for wine and oil on Tuesday and Thursday (some also permit fish on Tuesday and Thursday during this first period), and for fish, wine and oil on Saturday and Sunday. The second period is December 20th through 24th when the traditional fasting discipline (no meat, poultry, eggs, dairy, fish, wine and oil) is observed Monday through Friday, but with katalysis for wine and oil on Saturday and Sunday.
Taken from the Clergy Brotherhood email of the Diocese of Wichita and Mid-America.