The following is a version of something I post nearly every Advent, with a few relevant changes for the actual year.

Advent has started and with the start we also start the new Liturgical Year. Advent starts on the fourth Sunday before Christmas (or actually on the Saturday evening before that Sunday!). This First Sunday of Advent is the Sunday which falls on or closest to November 30. This year (2011) it is November 27, the earliest possible start!

Unlike Lent which precedes Easter and always has a constant length every year, Advent which precedes Christmas has a variable length. Easter itself (the day not the season) is always celebrated on a Sunday [once the Quartodeciman controversy in the Early Church was resolved]. However, Christmas is celebrated on a constant date, 25-December, which can be any day of the week. Thus, while there are always four Sundays of Advent, the actual fourth week can be quite short (when Christmas is a Monday) or it can be a full week (when Christmas is a Sunday).

Advent can thus vary from 22 to 28 days. This year (2011) Christmas falls on a Sunday. The fourth Advent week is a complete week! We thus have the longest possible Advent this year.

In the United States we celebrate Thanksgiving Day on the fourth Thursday of November. Since Thanksgiving Day is such a large secular holiday, it is a useful "warning" that we are about to start a new Liturgical year. In most years (five/sevenths of the time actually) Advent starts three days after the USA Thanksgiving. Sometimes there is an extra Sunday before Advent starts.

Now for some playfulness with the calendar. The following table shows the calendrical results. Note that Christmas always occurs 2 days earlier in the week (sometimes in the previous week) than November 1 since, for computational purposes, December 25 can be treated as November 55 (and November 57 is obviously the same day-of-week as November 1 since their dates differ by a multiple of seven). Note that only when November 1 is a Wednesday or a Thursday (thus making Thanksgiving relatively early in November) is there room in the calendar for an extra Sunday after Thanksgiving and before Advent.

Code:

Day of  Date of  Date of  Day of   Date    Date    Date     Date
Nov 1   1st Thu  Thanks.  Christ. 4 Adv.  3 Adv.  2 Adv.   1 Adv.
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Sun     Nov 5    Nov 26    Fri    Dec 20  Dec 13  Dec 6    Nov 29
Mon     Nov 4    Nov 25    Sat    Dec 19  Dec 12  Dec 5    Nov 28
Tue     Nov 3    Nov 24    Sun    Dec 18  Dec 11  Dec 4    Nov 27
Wed     Nov 2    Nov 23    Mon    Dec 24  Dec 17  Dec 10   Dec 3*
Thu     Nov 1    Nov 22    Tue    Dec 23  Dec 16  Dec 9    Dec 2*
Fri     Nov 7    Nov 28    Wed    Dec 22  Dec 15  Dec 8    Dec  1
Sat     Nov 6    Nov 27    Thu    Dec 21  Dec 14  Dec 7    Nov 30
   * = There is a Sunday after Thanksgiving and before Advent starts.

Edward Pothier