Our calendars are usually made up of 365 days, but every four years we add a day to make what is known as a 'leap year'.
That's today – 29th February!
The extra day is added to help synchronise our calendar with the Earth's orbit around the sun and the actual passing of the seasons.
The Earth's orbit takes 365.25 days to complete a solar orbit. It's the .25 that creates the need for a leap year every four years to keep everything in tune!
A leap year also brings the tradition of women proposing to men. The legend goes that in 5th century Ireland, St Bridget complained to St Patrick that women had to wait too long for men to propose.
St Patrick's response?
That women could propose on 29th February!