Eleven days after nine months before Christmas...

Over lunch the other day, I recalled seeing somewhere a slightly strange series of events behind the tax year's April 6 start date.

It turns out that (in pre-1752 England, at least) the tax year normally started at the same time as the numeric year, on March 25 [1]. This was nine months before Christmas Day, was therefore the usual candidate for the date of the Incarnation, and thus a good excuse for starting another year. Neither of these were New Year's Day, of course, since that was still (obviously!) January 1, being based on the traditional Roman laying out of the calendar.

1752 was of course the year England finally moved over to the new-fangled, and consequently rather more accurate, Gregorian calendar - only a hundred and seventy years after it had first been decreed by Pope Gregory XIII. However, fifteen hundred years of "more or less dead on" meant that things were reckoned to be ten days out by the time the Julian calendar's replacement was drawn up; and this had grown to eleven by 1752. In order not to have due dates come early when a week and a bit were skipped to catch up, the tax year moved forward to April 5 [2]. Just for good measure, when the Gregorian calendar reckoned the year 1800 as a non-leap year, the tax year shifted to April 6, just when it would have been had Caesar had still been running the calendar.

Fortunately, things had calmed down a bit by 1900, and as the next chance to stay in line with the old ways won't be until 2100, we'll presumably be staying with April 6 for the foreseeable futue.