Martius opened the earliest Roman calendar, a ten month year that ran from spring to midwinter and left the cold gap unnumbered, and it kept a first month character long after January displaced it: the numbered names of September through December still count from March. The month belonged to Mars, father of Romulus and Remus, god of war and also of the fields on which the season's work began. His dancing priests, the Salii, carried the sacred shields through the city in March processions, and festivals such as the Equirria and the Quinquatrus readied horses and arms for the opening of the campaigning season, the double duty of war god and field guardian visible in the month's whole program. Ovid, opening the third book of the Fasti, has Mars himself expound the month that bears his name.
March held new year status far beyond Rome. In England the legal year began on Lady Day, the twenty fifth of March, until 1752, and the liturgical logic of that date, nine months before Christmas, made the Annunciation a natural point for the year to turn. The Persian new year, Nowruz, is bound not to any Roman inheritance but to the spring equinox itself, and is kept at the moment of equinox by Zoroastrians, by the Baha'i faith as Naw Ruz, and across Iran, Central Asia, the Caucasus and the Kurdish regions.
The equinox also governs Easter: the rule descending from the Council of Nicaea sets it on the first Sunday after the first full moon on or after the twenty first of March, which is why March holds Easter in some years and hands it to April in most.
Sources FastiOvid with The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens