It depends on what Country and what calender was being used at the time. In the UK, the Gregorian Calender was introduced in 1752 and the new year began on 1st January...prior to 1752, the Julian Calander was in place with the new year beginning on Lady Day 25th March. So March was month 1, April month 2, May was month 3 and so on...
This is why dates prior to 1752 that fell between January 1st and May 24th are recorded using the dual date system e.g. 14th February 1750/1 (1750 being old style,1751 being new style)