January 1st is celebrated as the New Year's Day in most Western countries today. It marks the beginning of a new year for all those following Gregorian calendar.
It is the oldest of holidays started by the Babylonians some 4000 years ago. Their New Year fell on the first day of spring which was a logical time to begin a new year. It is a time of season change, new crops and blossom.
Romans continued this tradition but their calendar was changed several times before Julius Caesar established Julian calendar in 46 B.C. The new year began on January 1. However, Church opposed this festival for a long time and this is why Western countries have celebrated it only for the past 400 years. It is marked by fireworks, parties and New Year's resolutions.
Although January 1 is a public holiday in most countries, different people use different calendars (some based on lunar and some on solar cycle) thus observing the beginning of a new year at different times. Hebrew New Year falls sometime in September or October in Western calendar, Chinese New Year in January or February, Thai and Bengali New Year are celebrated in April, Ethiopian on September 11 and many people still celebrate New Year on the first day of spring, March 21.