Like the secular calendar, the Jewish calendar includes 12 months. The Jewish calendar is based on the lunar cycle, with each month beginning when the first sliver of moon becomes visible after the new moon.
The lunar cycle is approximately 29 1/2 days, so some months in the Jewish calendar have 29 days and some have 30. Nissan is the first month of the calendar and coincides with March and April in the secular calendar. The months that follow are, in order, Iyar, Sivan, Tamuz, Menachem Av, Elul, Tishrei, Mar Cheshvan, Kislev, Tevet, Shevat and Adar. All months except Tamuz, Elul and Cheshvan include holidays.