Advent starts on the 4th Sunday before Christmas. Advent can start from the 27th November (the longest it can be) to the 3rd December (the shortest it can be). Advent only starts on the 1st December when Christmas Day is on a Wednesday. For Orthodox Christians, depending on when Orthodox Christians celebrate Christmas (25th December or 7th January), Advent can start on the 15th or 28th of November as Orthodox Advent has a fixed number of days.