Why New Years?

This holiday has me thinking. It is a weird choice to celebrate it at the fairly arbitrary changeover of the Roman calendar. I'd understand more if it was celebrated on a solstice, as that would actually be at a time of meaningful position relative to the sun. But then the whole Roman calendar is pretty arbitrary. Why is a week seven days? Why do months have variable number of days? Why are there 12 of them? All just random decisions. I'd be much more in favor of a lunar calendar system. That would then be tied to non-arbitrary things. But then time is just a concept itself. There is only truly now. Everything else is chemical and electrical creations in our brain. The past is our memory, the future our imagination.