(My embarrassed thanks to Father Julian Green, who gave me the idea for a revised explanation that is far simpler than the one I originally gave).

There seems to be a lot of confusion about the way that the psalm weeks work over Christmas, so I thought it would be worth summarising the rules.

What I’m saying will apply only to days that do not have their own particular psalms. Since quite a lot of days at this time of year do have their own psalms, the weekly pattern is only visible through the gaps. Moreover, the Evening Prayer psalms for the entire week after Christmas are a simple repetition of those of Christmas Day.

• Week 1 of Advent starts on the Sunday that falls between 27 November and 3 December. It is psalm week I.
• Week 2 of Advent starts on the Sunday that falls between 4 and 10 December. It is psalm week II.
• Week 3 of Advent starts on the Sunday that falls between 11 and 17 December. It is psalm week III.
• Week 4 of Advent starts on the Sunday that falls between 18 and 24 December. It is psalm week IV.
• Week 1 of the year starts on the Sunday that falls between 7 January and 13 January. Weeks of the year are easy: week 1 is psalm week I, week 2 is psalm week II, 3=III, 4=IV, 5=I, 6=II, and so on.

So that leaves us with the days between 25 December and the start of week 1 of the year. The underlying pattern is very simple:

• Week I starts on the Sunday that falls between 25 and 31 December. (Before that, the psalm week is IV, left over from the tail-end of Advent).
• Week II starts on the Sunday that falls between 1 and 7 January. (Except that if the Sunday actually falls on 7 January, it marks the start of Week 1 of the year, so Week II never gets a chance to happen at all).

Here is how it all works out for 2007:

Religious calendar

Sunday 31 December – feast of the Holy Family.
Monday 1 January – solemnity of the Mother of God.
Tuesday 2 January – Week I.
Wednesday 3 January – Week I.
Thursday 4 January – Week I.
Friday 5 January – Week I.
Saturday 6 January – solemnity of the Epiphany.
Sunday 7 January – feast of the Baptism of the Lord.
Monday 8 January – Monday of Week 1 of the year, Week I.
Tuesday 9 January – Tuesday of Week 1 of the year, Week I.

Commercial calendar

Sunday 31 December – feast of the Holy Family.
Monday 1 January – solemnity of the Mother of God.
Tuesday 2 January – Week I.
Wednesday 3 January – Week I.
Thursday 4 January – Week I.
Friday 5 January – Week I.
Saturday 6 January – Week I.
Sunday 7 January – solemnity of the Epiphany.
Monday 8 January – feast of the Baptism of the Lord.
Tuesday 9 January – Tuesday of Week 1 of the year, Week I.

The pattern is a little hard to discern in 2007 because the Epiphany falling on a Saturday means that the days after the Epiphany have been squashed out of existence (a pity: they have some phenomenal prayers). This won’t happen again until 2018.