The Hebrew months correspond to lunar cycles. The beginning of the new moon is called Rosh Ḥodesh, literally, “head of the month.” Because a lunar cycle is 29½ days, some months are one day longer than others, and some have two days of Rosh Ḥodesh, while others have only one.
The Hebrew word for month, ḥodesh, is from the same root as that of ḥadash, “new.” In biblical times, Rosh Ḥodesh was determined by observing when the first sliver of the new moon appeared. Jewish calendars now indicate when Rosh Ḥodesh starts, and whether it is observed for one day or two.1
1. Adapted from A Guide to Jewish Practice, Volume 2—Shabbat and Holidays. The Guide may be ordered from the Reconstructionist Press.