From Wiki

The Julian and Gregorian calendars make no effort to enforce a correlation between the day of week and the day of month or year. There are 14 possible layouts to the Gregorian Calendar and it can be difficult to deduce which layout goes with which year. One solution to this problem is the perpetual calendar. These will cover a range of years and will align the weekdays correctly to a given month and year. We can't have you not knowing which day of the week 7 February 1964 was, can we? (No, no, look it up yourself...)