something to do with the amount of time the earth takes to orbit the sun, not overly sure sorry! i think.... is it that the earth takes more than 365 days, and the only way to catch up is to have the extra day??
A day really lasts 23 hours 56 minutes
So the Earth really takes 365 and a quarter days to orbit the sun
a leap year occurs every 4 years because those quarter day of a day makes an extra one every 4 years(February 29th) :-)
I wasn't aware there are lots of theories. The only explanation I've ever heard is that the earth's orbit takes around 365 and a quarter days so we create an extra day every four years. But because it isn't exactly 365.25 we have rules about not having leap years in century years (e.g.1800, 1900, 2100) except when are divisible by 400 (e.g. 2000)