Everyone is always trying to figure out when each Jewish holiday will happen on our civil calendar. The reason for this is that the Jewish calendar is determined by the revolution of the moon around the Earth as opposed to the civil calendar that sets the lengths of the months to fit within a regular 12 month year.
This makes the Jewish calendar harder to follow. There are approximately 12.4 lunar months in every solar year, making a 12-month lunar calendar 11 days shorter than a solar year and a 13-month lunar 19 longer than a solar year. To compensate for this, the Jewish calendar uses a 12-month lunar calendar with an occasional extra month of Adar, called Adar II, inserted before the regular Adar.
Our lunar month, Rosh Chodesh meaning head of the month, begins when the first sliver of moon becomes visible after the darkness of no moon. According to an ancient tradition, the holiday was a reward given to the women of Israel because they refused to surrender their jewelry for the creation of the golden calf (Exodus 32). Today, Jewish feminists have taken this holiday to add new rituals and meaningful celebrations for women. Women of the Wall come together to celebrate each Rosh Hodesh at the Western Wall in Jerusalem.
The year on the Jewish calendar represents the number of years since creation, calculated by adding up the ages of people in the Bible. This does not necessarily mean that the universe has existed for only 5700 years as we understand years, but rather marks the beginning of Judaism as we know it today. This year is 5778. Jews do not use the words A.D. and B.C. to refer to the years on the civil calendar because these refer to Jesus as the son of God. Instead we use the abbreviations C.E. (Common Era) and B.C.E. (Before the Common Era).