In a normal year, February gets very short shrift indeed. Whereas the other months are afforded the luxury of 30 or even 31 days to make their mark, little February must make do with a paltry 28. In order to make some small reparation for this, when an extra day is required, it is February which is given the honour of hosting it.

The occasional 'extra day' is caused by an astronomical mismatch between the length of time it takes for the Earth to rotate around the sun (a year) and the amount of time for the Earth to rotate on its own axis (a day¹). The ancients realised that the year does not divide into a whole number of days - in fact it is approximately 365 and a quarter days - so they introduced the concept of the 'leap year', adding a day to February one year in every four. In the calendar devised by Julius Caesar and adopted by the early Christians, if the number of the year AD divides by four, it is a leap year².

This reckoning was good enough for over a thousand years, but then in the late 16th Century the very clever (some would say finicky) Aloysius Lilius and Christopher Clavius recognised that the days had 'drifted' in the year since early times. That was because the quarter-day was also an approximation; in fact the extra part is nearer 0.2425 of a day. They proposed, with the support of Pope Gregory XIII, that a century year should only be a leap year when divisible by 400 - so 2000 is a leap year, but 1900 was not and 2100 will not be. The change to the Gregorian calendar resulted in ten days³ being skipped to get back in step with the time of Christ's birth. This corrected calendar, known as the Gregorian, is the one in use in the West today.

At least two remnants of the old Julian calendar can still be found in modern parlance. The Russian revolution of 1917 is referred to as the 'October Revolution', even though it began on 7 November. This is because the Russian Tsars continued with the Julian calendar for over three centuries, so in their terms the date was 25 October. Another standard-bearer for privilege and tradition, Cambridge University, was equally loath to update its practices and kept to its traditional term-times when the calendar was shifted, with the famous result that May Week⁴ is in June.

The date 29 February, 2000, could play havoc with computers. If a computer thinks it's dealing with the year 1900, not 2000, it will have a problem because 2000 is a leap year, but 1900 was not. Therefore, all data entries for 29 February 1900, will, in theory, be rejected (but not at h2g2).

If you were born on 29 February then you share your birthday with:

• Born 1468 - Pope Paul III (1534 - 1549).
• Born 1792 - Gioacchino Rossini - Italian opera composer who wrote The Barber of Seville and William Tell.
• Born 1896 - William A Wellman - director of the first film to win an Oscar (1928) Wings.
• Born 1904 - Jimmy Dorsey - clarinetist and bandleader.
• Born 1908 - Balthus - French painter.
• Born 1928 - Joss Ackland - British character actor.

And finally, on 29 February 1228, Scottish women were legally allowed to propose to their loved ones.

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¹ There are actually two ways of measuring a day. The normal one we use is the solar day, the period between one time and the next that the sun is at a certain point in the sky, which lasts 24 hours. Astronomers prefer to use the sidereal day, the length of time it takes for the Earth to rotate back to the same point relative to the distant stars. Because of the Earth's movement around the sun, there is one more sidereal day per year than solar day - meaning that the sidereal day is some four minutes shorter.
² The fact - so beloved of millennial pedants - that there was no year 'zero' means that figuring leap years going backwards is less straightforward. A BC year was a leap year if it is one more than a multiple of four.
³ From 5 to 14 October (inclusive), 1582.
⁴ The week of celebration which follows the end of the summer exams.