In the calender used by Westerners, why do some month have 30 days and some have 31 days. Was it arbitrary decided? Or has it got something to do with the moon? I do know why Feburary usually has 28 days but every 4 years, it has 29 days. It is to account for the fact that the earth needs 365 and a quarter days to spin a full turn around the sun. However, why choose Feburary to have that extra day every 4 years?