Here's my favourite category of fact: a date that never happened. If you had been in London in 1752, you would have gone to bed on Wednesday 2 September and woken up on Thursday 14 September. The third to the thirteenth simply were not issued. Under the Calendar (New Style) Act 1750, Britain and the whole empire skipped eleven days to fall in step with the Gregorian calendar, which most of Catholic Europe had been using since 1582. The same act moved the start of the legal year from 25 March to 1 January (which made 1751 a stubby little year of some 282 days).
So did mobs really pour into the streets shouting "Give us our eleven days"? Almost certainly not. The slogan survives on a placard in the corner of a Hogarth election painting from the mid-1750s, where it is a jab at a political opponent rather than a report from a riot. Historians have looked hard for the crowds and found mostly quiet grumbling about rent, wages and saints' days landing in the wrong place. Fair enough, really.
But the eleven days did leave a fingerprint. Move the old quarter day of 25 March forward eleven days and you get 5 April; add one more for 1800, which the new calendar declined to make a leap year, and you get 6 April. Which is why the British tax year still begins on a date chosen, indirectly, by a fortnight that was cancelled.
Pip, the reform left its mark in ink too: for years afterwards letters, ledgers and gravestones carried O.S. and N.S. — Old Style and New Style — so a birth could be dated twice without lying once. Almanack printers had the worst of it, having to set September 1752 with a gap in the column and a note explaining why the month was short.
Pip, the Act's drafters did the decent thing and legislated that rents, interest and taxes fell due only after the full natural time had passed — no one was billed for days that were never issued. Parliament rarely gets credit for tidiness, but that clause is a small monument to it.