We celebrate the solemnity of Mary because its is a sacred day of obligation and completes part of Christmas and new years day.
The Solemnity of Mary honors Mary as the Mother of Jesus including both His Humanity and His Divinity. The very early Church leaders did not believe that Mary, a human, could be the mother of Jesus...
New Year's Day (January 1st)
Because the glorious assumption of the blessed virgin Mary is a marvelous and wondrous thing. And we celebrate marvelous and wondrous things!
Roman Catholics celebrate her on September 8