By and large to Jews today is Rosh Hashanah, the beginning of a new year. To many others it is Yom Teruah, the Day of Trumpets. To some it is both. For certain, though, it's one of the feasts of the Lord described in Leviticus 23.
I wrote about this on another blog a few years ago. Here's the link:
Biblically, Rosh Hashanah occurred about six months ago, in the spring.
Ex 12:1-6
1 And the Lord spake unto Moses and Aaron in the land of Egypt, saying,
2 This month shall be unto you the beginning of months: it shall be the first month of the year to you.
KJV
Read on and we'll see that this is the month of the Passover, the month God brought Israel out of Egypt. No other month is spoken of as the beginning of the year anywhere in the Bible. So, the beginning of the first month, not the seventh, is Rosh Hashanah. Considering a seventh month year's beginning is something the Jews brought back with them from Babylonian captivity.