From Wikipedia

The Jewish calendar is a lunisolar calendar, or fixed lunar year, based on twelve lunar months of twenty-nine or thirty days, with an intercalary lunar month added seven times every nineteen years (once every two to three years) to synchronize the twelve lunar cycles with the slightly longer solar year. Each Jewish lunar month starts with the new moon. Although originally the new lunar crescent had to be observed and certified by witnesses, the moment of the new moon is now approximated arithmetically.
Concurrently there is a weekly cycle of seven days, mirroring the seven-day period of the Book of Genesis in which the world is created. The names for the days of the week, like those in the Creation account, are simply the day number within the week, with Shabbat being the seventh day. The Jewish day always runs from sunset to the next sunset; the formal adjustments used to specify a standard time and time zones are not relevant to the Jewish calendar.
The twelve regular months are: Nisan (30 days), Iyar (29 days), Sivan (30 days), Tammuz (29 days), Av (30 days), Elul (29 days), Tishrei (30 days), Marcheshvan (29 or 30 days), Kislev (29 or 30 days), Tevet (29 days), Shevat (30 days), and Adar (29 days). In the leap years (such as 5771) an additional month, Adar I (30 days) is added after Shevat, and the regular Adar is referred to as "Adar II".
The first month of the festival year is Nisan. 15 Nisan is the start of the festival of Passover, corresponding to the full moon of Nisan. Passover is a spring festival associated with the barley harvest,[3] so the leap-month mentioned above is intercalated periodically to keep this festival in the northern hemisphere's spring season. Since the adoption of a fixed calendar, intercalations in the Hebrew calendar have been at fixed points in a 19-year cycle. Prior to this, the intercalation was determined empirically:
The year may be intercalated on three grounds: 'aviv [i.e.the ripeness of barley], fruits of trees, and the equinox. On two of these grounds it should be intercalated, but not on one of them alone.[4]
The Bible designates Nisan, which it calls Aviv (Exodus 13:4), as the first month of the year (Exodus 12:2). At the same time, the season of the fall Festival of Booths (Sukkoth), is called "the end of the year" (Exodus 23:16). The Sabbatical year in which the land was to lie fallow, necessarily began at the time the winter barley and winter wheat would have been sown, in the fall.[citation needed] The Gezer calendar, an Israelite or Canaanite inscription c. 900 BCE, also begins in the fall.[5]
Modern practice follows the scheme described in the Mishnah: Nisan is the new year for the reigns of kings and the festivals. Rosh Hashanah, which means "the head of the year", and is celebrated in the month of Tishrei, is "the new year for the counting of years."[6] This is when the numbered year changes, which is most significant for determining the Shemittah and Yovel years.