for example, much of the English-speaking world celebrates some derivative of Armistice Day on 11 November, and a minority celebrates Martinmas
but you COULD be celebrating Old Halloween
(the Gregorian calendar, at time of introduction, was 10 days off from the Julian. the English-speaking world was so slow to it that it became 11. certain things, like tax days, kept their “old” dates by advancing them forward 11 days, so that people wouldn’t be swindled.
the 11 day offset was fixed in stone at this time. it does not accommodate further divergence between the calendar systems.)
works with any holiday, have fun