29th February
Every four years we get an extra day in February, but why February? Why not the end of the year? Well February used to be the end of the year.
Back at the time of the Roman Empire, February was the last month of the year. However, Julius Caesar reformed the Roman Calendar in 47 BC, introducing the Julian calendar, where he determined that the months of a year should alternate between 30 and 31 days. At 365 days a year, there were only 28 days left for the last month, February. And so it was logical to add the leap day to this last and shortest month. Since the introduction of the Gregorian calendar, December is the last month, but the tradition of adding a day to February in a leap year has been preserved.