Why, people often wonder, does the British financial year begin in early April, with the Chancellor of the Exchequer making his annual financial statement in March? It is all because of the reform of the calendar in the 18th century.
The ancient Roman calendar had only 355 days in a year, and was thus badly out of sync with the solar year. So in 46 BC Julius Caesar asked mathematicians to decide on the correct length of the year, and he then ordered a new calendar from their calculations. The Julian Calendar, as it came to be known, allowed for 365 and a quarter days in a year, with an extra day was inserted every four years to allow for this awkward fraction. Julius Caesar also decreed that each year should start on January 1st (a day of no solar significance, being several days after the winter solstice). Two extra months were inserted in the summer: with typical Roman modesty, one was named after himself (July) and the next after his successor, Augustus. (I am sure many people have noticed that the later months derive their names from the Latin words for seven, eight, nine and ten - septem, octo, novem and decem - whereas they are actually now the months nine to twelve).
The result became known as the Julian calendar, and later became used throughout Christian Europe, but different countries still started their year on a variety of different dates (as with the Chinese, Jewish and Hindu New Years now). In England it was traditional for the New Year to be on March 25th: Lady Day; the feast of the Annunciation, when the Archangel Gabriel appeared to the Virgin Mary. On this day all taxes and rents were due to be paid. However, by the 16th century, improved methods of astronomy led mathematicians to realise that the Julian calendar was slightly incorrect: the true length of the solar year being a few minutes shorter than previously thought. It only amounted to less than one day per century, but the calendar still needed reforming.
In 1582 mathematicians working for Pope Gregory XIII produced a new calendar; the Gregorian, which is still in use today. It took the date forward ten days, to allow for days lost since Caesar's time, and to deal with the awkward fraction of the day, it decreed there would not be a Leap Year day three century years out of four. (Therefore there was no leap year day in 1700, 1800 or 1900; there was one in 2000, but there will not be one in 2100, 2200 or 2300 - not that this will be of any concern to us!) Most of western Europe soon adopted the new calendar, but England refused, because it was "Popish" and therefore suspect, and instead stuck to the old calendar and the change of year on March 25th. This has always made it very difficult for historians when writing about, for instance, the Spanish Armada or the Duke of Marlborough's campaigns: which calendar are we dealing with? the Julian or the Gregorian?
Finally in 1751 Henry Pelham's government pushed through Lord Chesterfield's Calendar Act, which imposed a New Year on January 1st and also advanced the date eleven days to catch up with the continent (not ten, because of the unnecessary leap year day in 1700) The days between September 2nd and 14th in 1752 therefore never existed in England. This caused some trouble, with rioters shouting "Give us back our eleven days!" The Russians, however, stuck to the old calendar, which after 1900 was thirteen days out, so that the October Revolution of 1917 actually took place in November. In January 1918 the new Communist government adopted the Gregorian calendar, though the Russian Orthodox church refused to change, and so continued to celebrate Christmas thirteen days after the west, and Easter on a different day entirely.
The last trace of the old calendar in England is still found in the financial year, which even today begins on March 25th plus those missing days; which is why we continue to have the Chancellor's annual Budget statement in late March.