Why do christams be on a Friday an then next year it be on a saturaday?

Christmas is celebrated on December 25th each year, but the day of the week it falls on changes annually due to the way our calendar is structured. A regular year advances the day of the week by one day (since there are 365 days in a year, which is 52 weeks and 1 extra day), while a leap year, which has 366 days, advances it by two days. For example, if Christmas is on a Friday one year, the following year it will fall on a Saturday unless it is a leap year, in which case it would be on a Sunday.