The modern calendar has only been in use in England and its colonies (including the American ones) since 1752. In that year two changes took place. First, New Year's Day was moved from 25th March to 1st January. And second, the old Julian calendar was replaced by the modern Gregorian calendar. Because these two calendars were 11 days out of step, eleven days were removed from the calendar in September 1752 , so that Wednesday the 2nd was followed by Thursday the 14th.
But the British tax year always started on New Year's Day, 25th March. To avoid complications in assessing tax for a short year, the eleven days were added back into the tax year of 1752-53 (on the new calendar). In 1800, which would have been a leap year on the old system, but not on the new, an additional day was added to the tax year. This is why the British tax year now runs from 6th April - 12 days after 25th March.
Before 1752 the calendar year in England and Wales ran from 25th March to 24th March. In 1751 there were no months of January or February because the year ran from 25th March to 31st December only. The day before 25th March 1751 was 24th March 1750 and the previous month was February 1750.
To avoid confusion between the old and new style calendars before 1752, dates on this web-site between 1st January and 24th March are indicated in dual-mode. For example, 1st March 1750/1 is the 1st March which came before 1752. It was in 1750 (old style) but would have been in 1751 (new style) if the year had started on 1st January, as it does today.
There is one exception to this rule - dates in Quebec, Canada, formally known as Nouvelle France, or New France. Here, the French Catholic Church used in its records the French Calendar, which had converted to the new Gregorian calendar in 1582, and adopted January 1st as the start of the year in 1564 - both changes occurring before anything in Quebec or France on this website.
The new-style (Gregorian) calendar was introduced in different countries at different times. In Italy, France and many other catholic countries, it was introduced by Pope Gregory in 1582, with 10 days removed from the calendar. Protestant countries exerted their independence from the Pope by changing at various later dates. The change of New Year's Day to 1st January was also introduced at different times in different countries, often independently from the change to the Gregorian calendar. For example, in Scotland, the change of New Year's Day to 1st January was in 1600, but the change to the Gregorian calendar occurred at the same time as in England, in 1752.
For more information on this subject see the Wikipedia article on the Julian Calendar. A small free program called Calisto, which may be downloaded from here, shows the calendar in different European countries for any year from AD 1 to 2999, Want to know about the Ides of March? See this page on the Roman Calendar.
NOTE: All dates on this website are in the format of day / month / year.