Cyclical Gregorian kalendarya
The Gregorian calendar is full of wonderful mathematical quirks, and that's one of them: the beginning of the century, never fall on Sunday, Wednesday or Friday.
The reason is quite simple. We use the Gregorian calendar has a period of 400 years; if you add 400 years to any date, and day of the week it will be the same. This means that there are only four different days of the week, which may begin with the age, and it's Monday, Tuesday, Thursday or Saturday.