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5 Sundays in February????

In order for there to be 5 Sundays in February, February 1st must fall on a Sunday, and it must be a leap year, so that February 29th is the fifth Sunday. You agree? OK. Statistically speaking, a given date is associated with a day of the week one out of every seven years. For example, It is Friday today on 18th Jan, 2008. On 18th Jan 2015, it will again be Friday. Now, Leap years occur every four years. So, the probability of February 1st falling on a Sunday and it being a February of a leap year is 1/7 times 1/4, or 1/28. So, there should be 5 Sundays in February roughly every 28 years.
The American colonies were one of the last to adopt the currently used Gregorian calendar, in 1752. If we take that year as the start of the adoption of the Gregorian calendar, then there have been approximately 9 years with 5 Sundays in February. (What is Gregorian calendar, I'll explain in my next post, but for now you should know that the calendar which we are using today is Gregorian).
The last time there were 5 Sundays in February was 2004 and it was obviously a leap year. Before that, there were 5 Sundays in February in 1976, 32 years ago; similarly, after this year, there will be 5 Sundays in February in 2032, 2060 and 2088. So at least for the next few decades, the 28-year period I estimated above is exact. But for longer time in future, it may not be exact. Why? Wait for the next post. I'll explain.