Leap Year -- Day
Leap years were invented to make up the difference (about six hours) betwen the civil 365 day year, and the solar year which lasts 365.2422 days. Leap years have been traced back to Egyptian times, but it was Julius Cæsar and later Augustus who successfully introduced them into the Roman calendar by recognising two February 24ths. by a curious quirk of timing, any year whose date is a number exactly divisible by four is a leap year, except those years ending in '00', which must be divisible by 400 to qualify. As might be expected, a number of superstitions surround the leap year which many cultures traditionally regarded as unlucky. It was associated with bad harvests, barren marriages, tempestuous weather and ill-luck for monarchs. Yet all is not bleak: 29 February has long been considered the one day on which it is permissible for women to ask men for their hand in marriage.
So says my invaluable Schott's Almanac desk calendar. There is a mass of further calendrical information at the Wikipedia Leap Year site, some of it understandable even by the mathematically challenged, such as your servant.
And, of enormous significance, leap year is integral to The Pirates of Penzance. Young Frederick, having been apprenticed to a band of pirates, on reaching 21 years of age is out of his indentures and may now "renounce my vile profession." Or so he thought. But as the Pirate King explains:
For some ridiculous reason, to which, however, I’ve no desire to be disloyal,
Some person in authority, I don’t know who, very likely the Astronomer Royal,
Has decided that, although for such a beastly month as February, twenty-eight days as a rule are plenty,
One year in every four his days shall be reckoned as nine and twenty.
Through some singular coincidence – I shouldn’t be surprised if it were owing to the agency of an ill-natured fairy –
You are the victim of this clumsy arrangement, having been born in leap-year, on the twenty-ninth of February;
And so, by a simple arithmetical process, you’ll easily discover,
That though you’ve lived twenty-one years, yet, if we go by birthdays, you’re only five and a little bit over!
So you see how dicey leap year can be. So far as I can tell, using this go-by-birthdays rule, Frederick won't be able to get into his I.R.A. without penalty until he's 238 years old.