Why we celebrate new year on January 1 ?

Because Julius Caesar said so.

Early Roman Calendar

Since long before Caesar’s time, date keeping was dicey. In fact, the 355-day Roman calendar that immediately preceded Caesar’s Julian, worked on a four year cycle where every other year, an additional month was inserted between February (Februarius), the last month of that calendar year, and March (Martius), the first month of the year; this was done in order to catch the calendar up with the Earth’s orbit of the Sun. That additional month, called the Mensis intercalaris, brought in the missing 22 or 23 days, and to even things up, took another five days from February in the years it was present.

Since the calendar had been designed to ensure the proper observance of religious dates, priests, called pontifices, were responsible for declaring when the interclarismonth should begin and end. Since these priests were also involved with politics, they sometimes:

Misused their power by intercalating days or not intercalating them, merely in order to lengthen or shorten some magistrate’s year of office, or to increase the gains of some government contractor, or to inflict loss upon him.

By the time Caesar came around, the Roman calendar was in shambles, and in 46 BC, Julius Caesar commanded that it be changed.

Caesar set the New Year to January 1.  Why? Since 153 BC, January 1 was the day new consuls in Rome took office and Romans had commonly used the name of the two consuls to identify a specific year in question.  Thus, by officially making January 1 start the New Year, it simply lined up with the consular year.

As to why the consular year started on January 1 instead of the original Roman Calendar New Year’s day of March 1, this isn’t known.  That said, there are references that seem to imply that January 1 may have begun marking the New Year as early as 189 BC, which precedes when the consular year started beginning on that day.

One proposed reason for this switch is that January is thought by most to have been named after the god of transitions and beginnings, Janus, during the reign of the second King of Rome, Numa Pompilius, who lived from 753-673 BC.  Thus, it was naturally enough for the Romans to eventually decide to make the switch. However, whether this is the reason or not is very much up for debate.