Adam Clarke on Leviticus 23:24

A memorial of blowing of trumpets - This is generally called the feast of trumpets; and as it took place on the first day of the seventh month, Tisri, which answers to September, which month was the commencement of what was called the civil year, the feast probably had no other design than to celebrate the commencement of that year, if indeed such a distinction obtained among the ancient Jews. See the note on Exo 12:2. Some think creation began at this time.