Every year divisible by four. Such years occur every fourth year. In ordinary years the day of the month which falls on Monday this year, will fall on Tuesday next year, and Wednesday the year after; but the fourth year will leap over Thursday to Friday. This is because a day is added to February, which, of course, affects every subsequent day of the year. (See Bissextile.)
The story told above is of no historic value, for an Act of the Scottish Parliament, passed in the year 1228, has been unearthed which runs thus:
“Ordonit that during ye reign of her maist blessed maiestie, Margaret, ilka maiden, ladee of baith high and lowe estait, shall hae libertie to speak ye man she likes. Gif he refuses to tak hir to bee his wyf, he shale be mulct in the sum of ane hundridty pundes, or less, as his estait may bee, except and alwais gif he can make it appeare that he is betrothit to anither woman, then he: schal be free.”
N.B. The year 1228 was, of course, a leap-year.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894