A biblical calendar is a lunar calendar consisting of 12 lunar months of 29 or 30 days each. The earth's rotation on its axis measures a solar day, its annual solar orbit is a solar year, and the monthly phases of the moon indicate a lunar month.
The biblical calendar as used by the Hebrews is a bound solar or lunisolar calendar and depicts agricultural activity, festivals and seasons, as elaborated in the Bible. It begins with the month of Abib, or Nisan, in spring, and ends with the month of Adar. The additional month of Veadar, or the second Adar, is added periodically when seasons fall behind sufficient to require a significant calendar adjustment.