Israel just celebrated “Rosh Chodesh Nisan” – the beginning of the month of “Nisan,” April 4. "Rosh" mean head or top, "Chodesh" means month, so it is the "Head (first) of the month. Nisan is also the first month of the year on the Jewish calender.
Jewish months are based on a lunar calendar. In fact, the word “month” is derived from the word “moon.” The occurrence of Rosh Chodesh was originally based on the testimony of two witnesses observing the new moon, since this was before science understood the cycle of the moon.
For centuries, new moons had to be “sited” and new months had to be “proclaimed” in Jewish courts since there wasn't a Jewish calendar. And without a calendar there would be no Jewish holidays.
After observing the beginning slice of the moon, two witnesses appeared before the Sanhedrin and the day was declared as Rosh Chodesh. After declaring the new month, news of it would then be communicated throughout Israel and the diaspora.