In order to keep the calendar in line with the movement of the earth around the sun, every fourth year has 366 days instead of 365. The extra day is added after February 28.
The 366 day year is called a leap year, because after February 29, a date “leaps” over a day of the week. A date falling on Monday the year before leap year will fall on Wednesday in the leap year, instead of on Tuesday.
In parts of Europe, the custom arose that on February 29, a woman could ask a man to marry her. A legend tells that the custom began in Ireland during the fifth century. At that time, priests and nuns were allowed to marry. St. Bridget asked St. Patrick if nuns could propose to priests, and he agreed to allow the practice only every seven years. But she convinced him to shorten it to every four years, during leap years only.
This story is probably an invention, because Bridget was only about 8 years old when Patrick died. But we do know that a law passed in Scotland in 1288 not only gave a woman the right to ask for a man’s hand in marriage during a leap year but also levied a fine on a man who refused to accept such a marriage proposal!