Because it marks the first day of the New Year on the Gregorian calendar, just as the Fourth of July is on the Fourth of July in the US and the Winter Solstice is celebrated on the day of the Winter...
A New Day, the record label from England (Farnborough, Hants)
New Years Day is January 1st.
September 11th
Yes, January 1st is a Holy Day, the Feast of the Solemnity of Mary. http://www.cptryon.org/compassion/mary/trad04.html