Why not April 1st, or July 9th, or even October 31st?
When was it decided that January 1st was the beginning of a new year?
I answered a similar question a few days ago: Why is January 1 the first day of the year?
The short answer is that's when the Roman calendar started. But why did they choose January 1? We don't know, and it happened so far in their own past that the Romans who wrote about it also no longer remembered.