Calendar dates repeat regularly every 28 years, but they also repeat at 5-year and 6-year intervals, depending on when a leap year occurs within those cycles, according to an article from the Sydney Observatory. Repeating calendar dates are also affected by the adjustments made for leap years at the end of each century.
The calendar dates of leap years within the same century only repeat every 28 years, but the calendars of non-leap years repeat more often than that. For example, as shown by Timeanddate.com, the calendar for the year 2000 repeats every 28 years until 2084, but the calendar for 2001 repeats 6 years later in 2007, 17 years later in 2018 and 28 years later in 2028. The cause of these varying cycles of repetition is the fact that while the date usually advances by one weekday every year, it advances two weekdays on a leap year. This pattern can be illustrated by an example: The year 2000 starts on a Saturday, and without any leap years, the year 2007 would also start on a Saturday. However, since 2000 and 2004 are both leap years, the year 2007 starts two weekdays later, on a Monday. Any two years that are not leap years and begin on the same day of the week will share the same calendar dates.