What is New Year's Day?

Legal Definition
New Year's Day, also called simply New Year's or New Year, is observed on January 1, the first day of the year on the modern Gregorian calendar as well as the Julian calendar. In pre-Christian Rome under the Julian calendar, the day was dedicated to Janus, god of gateways and beginnings, for whom January is also named. As a date in the Gregorian calendar of Christendom, New Year's Day liturgically marked the Feast of the Naming and Circumcision of Jesus, which is still observed as such in the Anglican Church and Lutheran Church. In present day, with most countries now using the Gregorian calendar as their de facto calendar, New Year's Day is probably the most celebrated public holiday, often observed with fireworks at the stroke of midnight as the new year starts in each time zone. Other global New Years' Day traditions include making New Year's resolutions and calling one's friends and family.
-- Wikipedia
Legal Definition
The first day of January. The 25th of March was the civil and legal New Year's Day, till the alteration of the style in 1752, when it was permanently fixed at the ist of January. In Scotland the year was, by a proclamation, which bears date 27th of November, 1599, ordered thenceforth to commence in that kingdom on the ist of January instead of the 25th of March. Enc. Lond.
-- Black's Law Dictionary