Google reminded me this morning that today marks the 434th anniversary of the Gregorian calendar. A bit of digging around informed me that 明治5年10月1日（which is equivalent to November 1, 1872） saw the announcement that the Gregorian calendar would be adopted in Japan the following year. It must have been confusing - I mean, I get confused by daylight savings time, and that’s just one hour on a single day.
A beautiful surprise was that the numerically numb current Japanese names for months used to be much more poetic; we’re now in Kaminazuki (神無月, “Month of the Gods” - I know, 無 is a negative, but it’s complicated). Halfway down this Wikipedia page is a very informative list.
Below left is a calendar from 1729 (try and figure that out!) and right is an official pamphlet explaining the transition to the Gregorian calendar from 1872.