The Hebrew Calendar
The Hebrew calendar was composed of 12 lunar months, each of which began when the thin crescent moon was first visible at sunset. They were composed of approximately 29/30 days and were built around the agricultural seasons. Apparently some of the names of the months were accommodated from Babylon following the time that the Israelites were exiled there.
A comparison between the Hebrew and Gregorian Calendars can be viewed by clicking here.