In order to synchronize lunar and solar years, Jews added an extra month to twelve lunar months, as required. Before the Exile, the new year began in autumn when agricultural work was finished; but in the Exile the Jews took over the Babylonian year, which began in the spring, as the civil year, while retaining the former New Year for religious use.

The observance of Rosh Hashanah (‘beginning [literally ‘head’] of the year’) still takes place in September–October (the first day of the month Tishri). According to Jewish tradition, there was a period of God's judgement which began on this day and lasted until Yom Kippur ten days later.