January 1 is the New Year on the Gregorian Calendar, which most of the world uses now. However today is New Year’s Day on the Julian calendar introduced by Julius Caesar in 46 BC, which was, in turn, a revision of the Roman calendar.
Many countries, including Italy, Poland, Portugal, and Spain, accepted the Gregorian calendar in 1582. The conversion took longer for other countries such as Great Britain (1752) and Lithuania (1915).
Why so many calendars? Astrologically speaking, a year is the time it takes our planet to complete a full orbit around the Sun. It is approximately 365.242189 days and its length changes slightly over time.
Because the currently used Gregorian calendar has 365 days, a leap day is added to correlate it to the astronomical year. Without leap days, in less than 50 years, the March equinox would be in April and the June solstice would occur in July.
Is there a perfect calendar? The simple answer is no. None of the calendar systems currently in use reflect the astronomical year. However some calendar systems are more accurate than others.
The most accurate calendar currently used is the Persian calendar or the Solar Hijri Calendar, developed in the 11th century by a group of astronomers including the Persian scientist Omar Khayyam.