Did you know that for the first 130 years of Colonial America we celebrated New Year's Day on March 25th? It's true.
Britain at that time reckoned March 25th as the beginning of the new year. On this day rents were due, contracts began, and obligations were renewed. They based this on the Julian calendar - authored by Julius Caesar in 46 B.C. That calendar had 12 months and 365 days, but had one flaw. There was no allowance for leap years, so it didn't quite match up with the solar year.
Many European countries had already adopted the use of the Gregorian calendar - authored by Pope Gregory XIII. Britain, not wanting to follow a Pope's lead, clung to the Julian calendar until 1752.
Colonial America was part of the British Empire, but it was also already culturally diverse. Immigrants from across Europe made the colonists familiar with both calendars. When the official switch came in 1752, the colonists took it all in stride, avoiding the upheaval that happened across the pond, and showing once again the colonists unique ability to adapt and prosper.
A nice tidbit of history to think about when writing - or reading - books set in Colonial America.