Today is Quartidi, the 4th of Nivôse, Year 211. The French Republican Calendar was "a radical effort - it carried the ideals of the new republic directly into the daily life of every citizen. It was an artistic expression, an effort to make calendar names relate to the world of Nature, an attack on Catholicism (and religion in general), an effort to uplift and support a growing agricultural class, an attempt at decimal time and a basic functional calendar". New month names, new day names. What confidence! This was 1793.
In 1752 the British switched to the Gregorian calendar, remarkably just less than 200 years after the rest of Europe. Eleven days in September were skipped, and year numbering changed from being in March to January. Two consequences (that I heard on In Our Time a few days back): the financial year didn't change length, so the date of it had to change, hence it's now the beginning of April (the old year beginning, plus eleven days); whereas some festivals remained fixed relative to the seasons (Michaelmas is when apples ripen for example), other have fixed dates -- like Christmas, which is now earlier, and is why we associate Christmas with snow even though it doesn't get that cold until January.
Most impressively, the original Papal Bull switching to the Gregorian calendar (1582) and the original legislation introducing the French Calendrier Républicain (1793) are both online. The www is brilliant.