Find this year's anchors. Look up which day of the week 4 April falls on this year. Now check 6 June, 8 August, 10 October and 12 December. They are all the same day. These are the anchors. To find any date, start at the nearest anchor in that month, count the days to your date, take away sevens, and count on what is left.

Before chapter 4. What do the Saturday dates have in common?

The answer. The day and the month are the same even number: 4/4, 6/6, 8/8, 10/10, 12/12. In 2026 they are all Saturdays. Christmas is 13 days after 12 December; take away 7 and 6 are left, and Saturday plus six days is Friday.

The grown-up layer. Taking away whole sevens and keeping the leftover is clock arithmetic (modular arithmetic). The method is John Conway's "Doomsday rule". ShowMath has Wendel and Modo.

Pause to think. Why did Danya's 29 February not work?

For the ride home. Ask: what day of the week is your birthday this year?