New Moon celebrations are mentioned throughout the Scriptures. In Hebrew, the New Moon is called Rosh Chodesh. According to Vine’s Dictionary of Biblical Terms, the Hebrew word rosh means ‘head’ and chadash means to ‘renew’ in the sense of ‘refresh.’ The noun form of chodesh means ‘moon’ making Rosh Chodesh literally mean the ‘renewing of the moon’ at the beginning of each month. Using the calendar God gave to the Israelites, there are 12 months of 28 days in a year. Each month is determined by the lunar cycle and begins with the new moon; 14 days after the new moon, there is a full moon.
http://tentstakeministries.net/2010/11/new-moon-rosh-chodesh/