New Years Day
I'm not sure about other countries, but I believe that in the British Empire, including at the time the American colonies, new years day switched from April to January 1 when they switched from the Julian to the Gregorian calendar in (I think) 1757. This is why you frequently see dates in the earlier part of the 18th century written as, e.g., Feb. 11, 1737/38 indicating that it was still 1737 in the Julian calendar but 1738 in the Gregorian.