The problem is that we measure time in rotations of this planet; in other words, we divide time up into days. Unfortunately, the rest of the Solar System is not organised in multiples of this scale division. Our passage around the Sun is roughly 365 1/4 days so to bring the calendar of days back in line with our passage around the Sun, we'd need to stick in 1/4 of a day every year. That's nonsense because it doesn't fit in with the way we live our lives so best to insert a whole extra day every 4 years. It seems that leap year days in February go all the way back to Julius Caesar.
