Historically Rosh Chodesh was marked with a festive meal and the blowing of a shofar. Today Rosh Chodesh is commemorated with special prayers in synagogue, and in some communities, innovative celebrations.
Rosh Chodesh–which literally means “head of the month”–is the minor holiday that marks the beginning of every Hebrew month. The Jewish calendar, which combines both lunar and solar aspects, has 12 months (and 13 in leap years ); half of these months are 30 days long and half are 29 days.
At the end of months that have 30 days, Rosh Chodesh is observed for two days, on the 30th day of the previous month and the first day of the new month. After months that have 29 days, only the first day of the next month is observed as Rosh Chodesh.
Rosh Chodesh itself is a minor holiday, with relatively few observances, but there are several significant liturgical changes. An extra paragraph is added to the Amidah, and to the grace after meals. An abbreviated form of Hallel is recited, and a selection from the Torah about the Rosh Chodesh sacrifice is read. An additional service, musaf, is added after Shacharit.
Rosh Chodesh has long been considered a special holiday for women. Some say that this is because the women of Israel did not offer their jewelry for the creation of the Golden Calf. As a result, they were given Rosh Chodesh as a day when they could abstain from work. To this day, some women refrain from some forms of labor on Rosh Chodesh. Rosh Chodesh has long been a time for Jewish women to gather for a wide variety of activities, from reciting tradition liturgy, to sharing a meal, discussing Jewish ethics, and working for social change.
Material excerpted from http://www.myjewishlearning.com/.