Today, February 29th, is a leap day, a calendar system that started in 46 BC by Julius Caesar, adding one leap day every four years.
Julius Caesar featured here in a self-decreed minted coin.
Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth‘s year is slightly more than 365 days. In modern terms, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun — actually we now know this takes about 365.24219 rotations).
So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter!