Define 'News Years'

It is 2007 A.C.
The error within that is that 'A.C.' stands for After Christ. In which case, anytime before 1 A.C. was B.C. (Before Christ) So due to the fact that the time Jesus Christ was alive actually happened A.C. must define after Christ was born. But according to Christianity, Jesus Christ was born on December 25th. Hence The Mass of Christ or Christmas. But if A.C. is After Christ was born, and the new years starts on Janurary 1st, there are 6 days in which never happened (Dec 26th - 31st). And even so, if A.C. accounts to after the time of which Christ died. According to Christianity, Christ died on Easter which also leaves more then a couple months Although Easter is on a different day every year (in a cycle of roughly 7-8 years, depending on Leap Years). So what is it that defines a "New Year". And why are we celebrating it?

I'm sorry.
I am tired, I got a total of 3 1/2 hours of sleep last night.