New Year's Day
January 1st. The ancient Romans began their year in March; hence such words as September, October, November, December, meaning the 7th, 8th, 9th, 10th month, had a rational meaning. Since the introduction of the Christian era, Christmas Day, Lady Day, Easter Day, and March 1st have in turns been considered as New Year's Day; but since the reform of the calendar in the sixteenth century, January 1st has been accepted as New Year's Day, because it was the eighth day after the Nativity, when Jesus was circumcised (Luke ii. 21). (See New Style.)
The civil and legal year began March 25th till after the alteration of the style, in 1752, when it was fixed, like the historic year, to January 1st. In Scotland the legal year was changed to January 1st as far back as 1600; the proclamation was made Nov. 27, 1599.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894