Which week day will your birthday be this year? Or in 2020? When will you next be able to re-use your 2019 calendar? This set of tasks is about the maths of calendars, and is inspired by Matt Parker’s video ‘How many calendars are there?’

## Equipment needed

Worksheets, optional spreadsheet task.

## Curriculum Topics Covered

**Birthdays Task **and** How to Re-Use Calendars Task** – remainders, multiples, modular arithmetic. Aimed at KS3.

**How to Re-Use Calendars Spreadsheet Task** – This task is to re-create the spreadsheet made by Matt Parker for his video ‘How many calendars are there?’ (below). It is aimed at older students (and teachers!) with experience writing formulae in Excel, who want to learn how to make a spreadsheet that will tell you which calendar type (eg starts on a Tuesday and isn’t a leap year) to use for any given year. Involves IF and MOD functions in Excel.

Skip straight to **How to Re-Use Calendars Spreadsheet Solutions** to see the finished spreadsheet and show your class which calendar to re-use in the year 2322, for example.

See **Teacher Notes and Solutions** for more information.