The Nativity Fast commences on November 15th and continues
through Saturday, December 24th. This Fast is divided into two parts: The 1st 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, with katalysis (relaxation of the fast) for fish, wine and oil on
Tuesday, Thursday, Saturday and Sunday. The 2nd is December 20th through 24th
when the traditional fasting discipline is observed Monday through Friday with
katalysis for wine and oil on Saturday and Sunday.

There is also allowance for fish and/or wine and oil on certain feast days.  See the monthly calendar here.