In Exodus 12:1-2, at the beginning of the section that decrees Passover, God also tells Moses and Aaron that the month of Passover is the first month of the year: The Lord said to Moses and Aaron in Egypt, "This month is to be for you the first month, the first month of your year. ..."
https://www.chabad.org/library/article_cdo/aid/2164005/jewis...
It was also the first month of the Roman calendar until January and February were added.