Had the new year begins in March, what happened?

December 25 is actually the beginning of spring is marked by the appearance of sunlight on the horizon south of Europe, and more festive celebration after a week later, ie on 1 January. That is beginning to move the date / month of new year

Gregorian calendar system is a dating system based on pseudo-cycle of movement of the Sun through the vernal point equinok two times in a row, the average length is 365, 242 199 days. Earth's revolution around the Sun each year resulting in the Sun visible from Earth moves across the sky ball. And, in fact the Earth moves around the Sun, we see the Sun is projected on a star field is different. Sun trajectory is apparent for a year and then called the ecliptic. Easily, just imagine the stars in the sky. The stars seemed rises and sets every day. It is none other than caused by rotation of the Earth about its axis, not because the Earth is quiet and surrounded by stars, like the thought of old people for centuries.

There's a very good description of the history of the Roman calendar system here: http://www.webexhibits.org/calendars/calendar-roman.html (be warned ... It's VERY complicated!) January and February were said to have been added to the original 10 months by Numa Pompilius in about 713 BCE and January, apparently, then became the first month of the year instead of March, though some sources claim that it became the first month under the Decemvirs in about 450 BCE. Julius Caesar (in about 46 BCE), when he completely revised the calendar, also proposed that January should be the first month of the year. There seems to be a lot of confusion about when January became the first month of the year (another page says that it was in 513 BCE but doesn't quote the source. I think that this may be a mistake for 713 BCE), but, whichever one you take, it was clearly long before the current Gregorian calendar was instituted and long before the birth of Christ. It is generally accepted that the anniversary of the birth of Jesus is NOT December 25th (and certainly not in the year from which we number our years now). It is likely that Christmas day was set arbitrarily in order to 'Christianise' a midwinter festival (Saturnalia), probably with something like this reasoning: "If the common people traditionally celebrate at this time, then let us give them something Christian to celebrate. Jesus is the Light of the World, so let us symbolically celebrate His birth when the pagans also celebrate the rebirth of the Sun (i.e. just after the Winter solstice, when the days are beginning to get longer)."

Sosigenes then deliver a one-year calendar calculations based on the time required by Earth for a time around the Sun. He said, the old Earth is 365.25 days circling the sun. However, so easy, fulfilled to 365 days. The drawback would be combined into 1 day every four years. That is the leap year! And in the years 45 BC, was the official dating system used by all the colonies of the Roman Empire. Then what date that will only appear once a few years? Also determined that one day will be added in February every leap year. Because February has the least number of days, namely 29. When leap year comes, February has 30 days.