Before 1752, the year began on Lady Day, 25 March and ended on 24 March. Earlier documents dated between 1 January and 24 March may be referred to with two years; for example, 1 January 1745/6.
In 1752, England adopted the Gregorian calendar which the rest of Europe had been using for almost two hundred years. In 1582, when Europe had changed from the Julian calendar that had been introduced by the Romans, Pope Gregory decided to remove the excess days that had been accumulated by the inaccuracies of the Julian calendar. When England adopted the Gregorian system of dating, the same adjustment had to be made.
So it was that in 1752 for the first time the year began on 1 January. In order to adjust the difference between the Julian and Gregorian calendars, 2 September was followed by 14 September. From then onwards England and the rest of Europe have shared the same calendar.