Brief explanation: March 25th used to be New Year's Day.
Short explanation:New Year's Day used to be March 1st (thanks to some Romans), but in 45 BC was moved to January 1st (thanks to a Roman), and in Anglo Saxon Britain changed to December 25th (thanks to some Roman Catholics), then in 1087 switched to January 1st (thanks to the Normans), then in 1155 moved to March 25th (thanks to some Roman Catholics), until it was finally moved back to January 1st in 1752 (thanks to further Roman Catholics).
Proper explanation:(deep breath, and let's hope not too much of the following is incorrect)
Before there was a calendar, springtime heralded the new year. For the Mesopotamians 4000 years ago, the new year started at the spring equinox.
In the book of Exodus, at the time of the first Passover, God tells Moses "This month is to be for you the first month, the first month of your year". The Jewish ecclesiastical new year still begins in the month of Nisan, on Nisan 1 (which in our calendar can fall anywhere from late March to early April).
In the earliest version of the Roman calendar there were only ten months, the first of which was March. New Year's Day was thus the calends of March, or March 1st. Later two extra months were added - which we now call January and February - but at the end of the year, not the start.
In 45 BC Julius Caesar introduced the Julian Calendar, switching the first day of the year to January 1st. This was the date on which senators started their annual terms of office, and had been since 153 BC (prior to which it had been March 15th). Julius's change also had the effect of nudging all the months two positions later than they'd originally been. Evidence: September, October, November and December are named after the Latin words for 7, 8, 9 and 10 but are now the 9th, 10th, 11th and 12th months.
After the fall of the Roman Empire, the Church started to take control of the calendar, switching the start of the year to match important Christian festivals. In 567 the Council of Tours recommended that the calendar year begin at Easter. By the 7th century several European countries preferred 25th December - Christmas Day. By the 9th century the preferred date across southern Europe was March 25th - the Annunciation of the Virgin Mary, or Lady Day.
England gravitated towards December 25th as the first day of the year, but not exclusively... in some places March 25th won out, and in others even September 24th. Then the Normans invaded. They didn't like national inconsistency, so in 1087 decided to standardise the new year by picking January 1st instead. It didn't last.
In 1155, immediately following the accession of Henry II, the Catholic church's preferred date of March 25th was imposed instead. And that stuck. For the next 596 years, right up until 1751, Lady Day would be the official start of the civil year in England. Had you been alive in 1219, 1319, 1419, 1519, 1619 or 1719, today would have been New Year's Day.
The beginning of the end for March 25th came in 1545 when the Roman Catholic church finally decided to tackle the shifting date of Easter. Julius Caesar's leap year calendar had only been out by 11½ minutes per year, but those 11½ minutes now added up to ten days and the seasons were in the wrong place. It took a while for astronomers and papal authority to agree how to proceed, but an early change was the blanket adoption of January 1st as the first day of the year in Catholic European countries.
The new Gregorian Calendar was finally introduced in 1582, ditching ten days from the calendar and introducing a new leap year rule concerning years ending in 00. But England was no longer a Catholic country, Henry VIII having seen to that, so changed nothing and continued using March 25th as the first day of the year.
Scotland, as an independent country, chose to follow Europe and adopted January 1st as the first day of the civil year in 1600. This worked fine until 1603 when the death of Queen Elizabeth united England and Scotland, after which the legal years north and south of the border officially began on different dates.
Great Britain remained stubbornly out of sync with the rest of Europe for the next century and a half, with legal documents, taxation and parliamentary business sticking rigidly to a year commencing March 25th. But in civilian life the idea of January 1st as the first day of the year crept inexorably into common usage, and chronology became increasingly messy.
For example, the execution of King Charles I took place on January 30th in a year we would now call 1649. But the death warrant is dated 'January xxixth Anno Domini 1648', because the year 1649 hadn't yet officially begun and 1648 still had two months to run.
To avoid ambiguity dual dating was introduced, with dates in January, February and March defined as either Old Style or New Style according to whether the British or continental system was being used.
For example, the accession of Queen Anne officially took place on 8th March 1701 (Old Style) which was simultaneously 8th March 1702 (New Style). A simple shorthand way of writing this in everyday life was to use a hyphen, i.e. 8th March 1701-2.
Eventually Parliament bit the bullet and passed the Calendar (New Style) Act 1750. This had two parts, firstly officially adopting January 1st as the first day of the year (instead of March 25th), and secondly adopting the Gregorian calendar (by skipping eleven days).
The Act was introduced into Parliament during what was officially February 1750, and received Royal Assent three months later in May 1751. America was still a British colony at the time, so the change applied there too, and across the wider Empire.
As an indication of the awkward consequences of this action, the years 1750, 1751 and 1752 were officially each of very different lengths.
1750: March 25th - March 24th (365 days) [the last normal Old Style year] 1751: March 25th - December 31st (282 days) [the shortest year ever] 1752: January 1st - December 31st (355 days) [a leap year, but eleven days short] 1753: January 1st - December 31st (365 days) [the first normal New Style year]
It would have caused financial chaos if the tax year had shifted to January 1st, with tenants paying too much in the shortened years, so this had to be left alone. However, even this wasn't straightforward. The loss of eleven days in 1752 meant the tax year had to be extended by eleven days to remain the same length, so the start date duly shifted from March 25th to April 5th. In 1800, which was the first century year not to be a leap year, the Exchequer nudged the start date ahead one more day. But they didn't nudge it in 1900, and have never nudged it again, which is why the tax year has begun on April 6th ever since.
Meanwhile Britain's new year has remained on January 1st since 1752, making this the 268th time that March 25th hasn't been New Year's Day. If only we'd held out against Europe a little longer, with their expert science-based ways, today could still have been a day of stubborn drunken celebration.