This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
That depends. Jews have 4 different days to choose from:
1 Tishri: "Rosh HaShanah". This day is a celebration of the creation
of the world and marks the start of a new calendar
year. This will be the day we shall base our calculations on
in the following sections.
15 Shevat: "Tu B'shevat". The new year for trees, when fruit tithes
should be brought.
1 Nisan: "New Year for Kings". Nisan is considered the first month,
although it occurs 6 or 7 months after the start of the
1 Elul: "New Year for Animal Tithes (Taxes)".
Only the first two dates are celebrated nowadays.