Stir-up Sunday used to be the 25th week after Trinity Sunday and hence was a moveable date tagged to Easter. The Church preferred this date be linked to Advent, and it is now set as the Sunday before Advent (as Advent is four Sundays before Christmas, it follows that Stir-up Sunday is the fifth Sunday before Christmas)

Thus, Stir-up Sunday for 2008 is 23rd November. However, an awful lot of folk out there are using 30th November as the date of Stir-up Sunday.