Who is the god of beginnings in ancient Roman religion?

Actually the old calendar didn't include January and February. that is why October which is named meaning 8 but is now the 10th month and September meaning 7 is now the 9th month.

Janus...January...beginnings

Gem-kat1, actually, April 1st was New Year's Day until the adoption of the Julian calendar, which is why April 1st is April Fools Day, for those who stubbornly followed April 1st for new years

Janus as they connect in the month of January has 2 faces the past and the future (am jan born)

Gem-kat1, like it

And Novem and Decem, Julius, Augustus, Mars, etc. All before th Gregorian calendar.