What is a ‘leap year’?

A LEAP YEAR is any year in which an ‘intercalary’ day exists–that is, a day that is inserted into the calendar to account for the tiny shortages of seconds, minutes and hours that occur across a given period (totalling 6 hours, specifically) caused by the fact that the Earth does not rotate around the sun precisely in 365 days. In the case of ‘our’ calendar, leap years are declared every four years (because 4 x 6hours = 24hours/1 day)
The phrase ‘leap year’ refers to the fact that in a regular year, any given date will advance by one day per year between one year and a next, but in a leap year that date will ‘leap’ over its proper day and come to rest on the next one.
Simple right?
Mmmm…not so much. Over a long period, those extra leap days, themselves, add up and so the Gregorian calendar accounts for that by removing three leap days every 400 years (sheesh, sure hope someone has made a note of that for 2400AD!) by NOT having a leap day in the three century years within any 400 year period that cannot be exactly divided by 400 (2100, 2200, 2300 in the current case).
What? Just…what?
The Celts seem to have liked things a bit simpler. The Gregorian (1582) Calendar wasn’t yet developed and the Celts refused to use the Julian (Roman) calendar that was (literally) in force over Rome’s Empire. Privately, at least. They used their own calendar for tracking the passage of time and their most significant observances. And it was a ‘lunisolar’ calendar (meaning the phases/seasons are determined using both sun and moon as a guide unlike our calendar which is purely solar) which puts it in the company of some traditional calendars you might recognise from today (Hebrew, Buddhist, Hindu, traditional Chinese, Japanese, Korean) some other ancient ones (Hellenic & Babylonian).
The Celtic ‘Coligny Calendar’ (for the location in France it was uncovered) at first looks completely baffling until you really get to know it and then it makes as much (or as little) sense as ours. To them, adding a single day in the second month of the year every fourth year and then robbing certain century years of it might seem pretty random (and difficult to track). Well the Celts didn’t mess about with a day here and there. Their core period of time was five years and they whacked an intercalary month in there at 2.5 years and the 5 year mark to bring everything back into reasonable alignment. That also seems a little random until you realise that was month 30 and 60. The dead middle and the end. Pretty logical really.
I’ve written a post about the Coligny Calendar here if you’d like to learn more about it.
Other interesting facts about leap years.
• A person born on Feb 29th is called a ‘leapling’
• The Olympic Games (summer) have been held only on leap years since 1896 (so, yes, we have one this year to be held in Rio)
• In Britain and Ireland, traditionally, women may propose marriage only in a leap year (although this ‘tradition’ doesn’t really trace back further than the 19th C)
• In the medieval period the leap day was calculated slightly differently and fell on or around Feb 24th (ie: the start of the last week of Feb).