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With the end of the tax year coming up, maybe it’s occurred to you to wonder why it falls on such a strange date. It actually all goes back to 1752. Or maybe to 1582. Or even to 46 BC.

In 46 BC, Julius Caesar devised the Julian Calendar – or, more realistically, had it devised for him. The traditional Roman calendar wasn’t very accurate, resulting in the official date being nowhere near where the astronomers said it should be, and Caesar introduced a system very similar to the one we use today.

There were two main differences: the date of New Year’s Day and a slight inaccuracy in the leap-year cycle. Various dates were tried for New Year, but the one eventually settled on was 25th March. That might seem on odd choice, but it more or less marks the Spring Equinox (in the same way that 25th December more or less marks the Winter Solstice) and as Lady Day is one of the four Quarter Days, along with Midsummer, Michaelmas and Christmas.

That slight inaccuracy was crucial, though. Since a year isn’t exactly 365¼ days, after 1600-odd years the Julian Calendar was again out of sync with what the astronomers observed. In 1582, Pope Gregory XIII “devised” (in the same sense as Caesar) a reformed calendar that addressed this issue, omitting three leap-years in every four hundred years, as well as moving New Year’s Day to 1st January.

This was a Catholic reform, though, so of course Protestant England ignored it till 1752, by which time it was clear we were out of step with most of Europe. In that year, besides New Year’s Day being changed, eleven days were omitted – leading to riots by people who thought the government had taken away eleven days of their lives.

The taxman, however, blithely ignored all of this. It was unthinkable that a tax year should be only 354 days, let alone the 270 that would have taken it to the beginning of January. So, exactly 365 days after 25th March 1752, the next tax year began on 5th April 1753.

Wait – 5th April? Well, the first time the expected leap-year didn’t happen, in 1800, the date was moved on another day to make the year start on the 6th. This didn’t happen in 1900, though, so ever since the tax year has begun on 6th April.

Which most people don’t care about. They just want to an accountant who’ll get them out of paying as much as possible.

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