First of January is the first day of the New Year on the modern Gregorian calendar, as it was also in ancient Rome (though other dates were also used in Rome).
In all countries using the Gregorian calendar as their main calendar, except for Israel, it is a public holiday, which is often celebrated with fireworks at the stroke of midnight as the New Year starts. January 1 on the Julian calendar corresponds to January 14 on the Gregorian calendar, and it is on that date that followers of some of the Eastern Orthodox churches celebrate the New Year.