An old version of this page said:
The Gregorian calendar also fixed the first day of the year as January 1, which was already the first day used in Italy, Germany, and other places, but not universally (the British Empire, for example, began the year on March 25). This is why the months named after the Latin numbers 7, 8, 9, and 10 (September, October, November, December) are now the 9th, 10th, 11th, and 12th month, respectively.
I have removed this last line, because it is incorrect. The shift from March 1 to January 1 as the first day of the year was already made when the Julian Calendar was installed.
Isn't there something where rulers expanded the number of days in a year to add more festival days and therefore the calendar did not match the changing of the seasons?
When did the shift to Jan 1 as the first of the year occur. Presently the Gregoian entry claims it and the Julian doesn't mention it. Is this true or not?
I'm afraid that there's no simple answer - it was irregular in different parts of Europe -- much like some corporations still operate on different fiscal years from others, the first of the year was not inflexible. Lots of medieval and renaissance historians have to double-date events (814/5) because it's not always entirely clear when some people in the past rolled over their year counter.