The Nativity Fast is a period of abstinence and penance practiced by the Eastern Orthodox, Oriental Orthodox, and Eastern Catholic Churches, in preparation for the Nativity of Christ, (December 25). The fast is similar to the Western Advent, except that it runs for 40 days instead of four weeks. The fast is observed from November 15 to December 24, inclusively. These dates apply to those Orthodox Churches which use the Revised Julian calendar, which is identical to the Gregorian calendar. For those national Churches which still follow the Julian calendar (Russia, Georgia, Ukraine, Macedonia, Montenegro, Serbia, and the Patriarchate of Jerusalem), the Winter Lent does not begin until November 28 (Gregorian) which coincides with November 15 on the Julian calendar.
Sometimes the fast is called Philip's Fast (or the Philippian Fast), as it traditionally begins on the day following the Feast of St. Philip the Apostle (November 14). Some churches have abbreviated the fast to start on December 10, following the Feast of the Conception by Saint Anne of the Most Holy Theotokos.
(... from Wikipedia on 2012-04-17 21:51:51)