Once again, we come to the great parting of the ways between Orthodox believers on the "Old Calendar" and those on the "New Calendar." The 13-day lag, of course, is with us year-round, since the fixed calendar never really stops. But for those blessed 18 weeks--more than a third of the year--we all share in common the movable feasts and fasts.
The movable calendar starts, at a minimum, ten weeks before Pascha and extends eight weeks after. The four Pre-Lenten Sundays include: Publican and Pharisee, Prodigal Son, Last Judgment, and Expulsion from Eden. Fasting is forbidden during the first week, regular during the second (ending with Meatfare Sunday), and vegetarian during the third (ending with Cheesefare Sunday).
Lent starts with Clean Week, the Monday after Cheesefare. Strict fasting continues for 40 days, including five Sundays: Triumph of Orthodoxy, Gregory Palamas, Exaltation of the Cross, Ladder of Divine Ascent, and Mary of Egypt.
The 40-day fast of Lent ends on Friday, followed by Lazarus Saturday, Palm Sunday, and the even stricter fast of Holy Week. Pascha follows on Sunday, then Bright Week, with fasting forbidden. Normal fasting resumes after Thomas Sunday, the first of six Sundays identified between Pascha and Pentecost: Thomas, Myrrhbearing Women, Paralytic, Samaritan Woman, Blind Man, and Holy Fathers.
The Feast of Ascension always falls on the Thursday before Holy Fathers, exactly 40 days after Pascha. Pentecost follows, on the 50th day after Pascha, then fasting is forbidden for a week until the Sunday of All Saints. That Monday starts the Apostles' Fast, which lasts until the Feast of Peter and Paul on June 29. This fast bridges the gap back to the fixed calendar and thus marks the end of the movable feasts. It's always 13 days longer on the Old Calendar than the New, and the rest of the year is just more of the same.
At least we have this much in common, but it still seems a high price to pay so we can celebrate Christmas on Western time.