On Saturday, Feb 29, 2000 was a leap day – a calendar oddity gave us an extra day. The time it takes earth to rotate on its axis is called a day – but it takes 365.2422 days to loop around the sun. That means when the clock strikes midnight on New Year Eve, earth has not quite circled all the way back to its starting point. So every four years, the month of February has 29 days instead of 28. Because the rotation of earth around sun is not exactly 365.25 days, to compensate this the years 1900, 2100 and so on will not have leap days. On Mars, a year lasts 668.6 Martian days. To keep the calendar in harmony with the Martian seasons, it was proposed that the even-numbered years have 668 Martian days, and odd-numbered years have 669 Martian days. Also to maintain similarity to Earth’s calendar, Mars retained 12 months with added extra ten months (each 30 or 31 days) named after famous astronomers, mathematicians and science-fiction writers.
https://www.sumeetsud.com/sumeet-sud-shares-information-about-mars-leap-days/