Question What are the Biblically ordained festivals, and how many days are they celebrated?
Answer Besides the Sabbath, there are five festivals ordained by the Torah. For reasons we will explain later, those living outside Israel are often required to keep extra days in addition to those ordained. The festival days are:
Rosh Hashanah - 1 Tishrei (September-October) 2 days
Yom Kippur - 10 Tishrei (September-October) 1 day
Sukkot/Shmini Atzeret - 15 Tishrei (September-October) 9 days (8 days in Israel)
Pesah (March-April) 8 days - 15 Nissan (7 days in Israel)
Shavu'ot - 6 Sivan (May-June) 2 days (1 day in Israel)
Question Why is there a difference between the number of days the latter three festivals are celebrated in Israel and the number of days they are celebrated elsewhere?
Answer In ancient times, there was no fixed calendar as such. Each month began with the sighting of the new moon. As the lunar cycle is about 29 ½ days, the sighting of the new moon could only take place on the 30th day or the 31st day after the previous new moon had been observed. Now, people who had sighted the new moon would come to the Sanhedrin - the supreme rabbinic court - in Jerusalem and would testify to this effect. If the testimony of two witnesses was deemed reliable, the beginning of a new month would be proclaimed, and the festival/s of that month would be scheduled accordingly. Messengers were then sent out by the Sanhedrin throughout the Land of Israel to inform all the communities when to celebrate the festival/s of that month. A problem arose, though, for communities situated too far for the messengers to arrive in time. These communities (i.e., those outside the Land of Israel) were never sure as to which day the new month had been proclaimed. As a result, they kept an additional day for each festival. Of course, nowadays we have a standard calendar, but the custom of an added day for the different festivals has remained in effect to this day.
Question Why then is there no difference between the number of days Rosh Hashanah and Yom Kippur are observed in Israel and elsewhere?
Answer According to the Torah, Rosh Hashanah is only one day. As it occurs on the first day of the month of Tishrei, the day that the new moon was sighted, even the Jews in the Land of Israel were unable to find out in time when the new month had been proclaimed, so they too kept two days. As to Yom Kippur, where those outside Israel should in theory have kept two days, the rabbis felt it was too difficult to expect people to fast for two days in a row (although there have been cases of extremely pious Jews who would do so).
Question What system was used for leap years before the current Jewish calendar went into effect?
Answer As prescribed by Torah law, the highest court in the Land of Israel was the one to determine when each new month began. (As we pointed out in the previous answer, the lunar cycle is 29 ½ days, so the new moon might appear on either the 30th or the 31st night after the last new moon.) Now, a 12 month lunar year would mean a loss of 11 days yearly. As the Torah specifies that Passover must occur in the spring, the Sages would go out into the fields each year, some time before the spring, and if the trees had not blossomed by a certain date they would declare the year a leap year, thus adding an extra month before Passover and ensuring that Passover would occur in the spring.
Question What is Rosh Chodesh?
Answer The words Rosh Hodesh simply mean "the head (i.e., beginning) of the month," and refer to the beginning of each new month. Rosh Hodesh is considered to be a minor type of festival, with certain modifications in the daily prayer service and grace after meals to indicate this. As we noted above, a Jewish month may be either 29 or 30 days long. Whenever a month is 29 days long, the following day - the first of the new month - is celebrated as Rosh Hodesh. When a month is 30 days long, two consecutive days are celebrated as Rosh Hodesh: the last (30th) day of the previous month, and the first day of the present month.