The number of days in one year is not equal to a whole number of weeks times seven days a week, so it takes some time for a person's birthday to fall on the same day of the week as the actual day (s)he was born on. Every fourth year is a leap year*, and every week has seven days. Since 28 is the Least common multiple for four and seven, it takes 28 years for the dates in a person's own calender (life) to start repeating themselves, although there are a couple of very close approximities every now and then.
ALL HAIL Milky, FOR STARTING ALL OVER AGAIN!!!!!
* Yes, I know that after Gregorian calender years 1700, 1800, 1900, 2100... are not leap years, but I really don't reckon any of the teotians is older than 112, or expects to live up to year 2100 (except maybe some of the deejay's offspring)!