According to The Mishna (Tractate Rosh Hashanah 1:1) there are 4 months in the Jewish year which begin the annual cycle for various aspects of Jewish life.

It was important to fix dates such as these for civil and political purposes, and for the implementation of regulations related to agricultural produce.

| First of Nisan |
The New Year for Jewish Kings and for the religious calendar.
If a king ascended the throne during the month of Adar (preceding Nisan), the next month would constitute the second month of his reign. (Also the New Year for the purchase of congregational sacrifices and renting of houses.)

| The First of Elul |
The New Year for the tithing of cattle.

| The First of Tishrei |
Rosh Hashanah
The New Year for the Civil Calendar, for the sabbatical and jubilee years, and for the year of planting fruits and vegetables. The establishment of the first of Tishrei as the religious New Year was based on the statement that on that day "all the world is judged " (Babylonian Talmud, Rosh Hashanah 1:2).

| The First of Shevat |
The New Year for Trees.
This is according to Bet Shammai. According to Bet Hillel, this new year falls on 15th Shevat, Tu B'Shvat. This is indeed the date celebrated today. On this date, the greater part of the year's rain has fallen.