Question Why do Jewish holidays occur on different calendar dates each year?

Answer Unlike the standard calendar in use throughout the western world, which is based purely on the solar cycle of a 365 day year, the Jewish calendar is based on both the solar and lunar (moon) cycles. The Jewish month begins with the new moon, and ends when the moon cycle terminates. The lunar cycle is about 29 ½ days, so - with rare exceptions - the Jewish months alternate between 29 days and 30 days in length. A Jewish year consists of 12 months, but if we multiply this by the average of 29 ½ days per month, we get 354 days, or 11 days less than a solar year of 365 days. In order to ensure that the solar and lunar years are synchronized, Judaism adds a full month every few years. In the 19 year cycle that is used, there are 7 leap months, so that leap years occur every two or three years, based on the year of the cycle. The extra month is added after the month of Adar, and is known as Adar II or Adar Bet. Now, as the Jewish year consists of either 353-355 days in a standard year and 383-385 days in a leap year, we can understand that any given Jewish date will come out on different calendar dates in any two consecutive years - either about 11 days earlier than the previous year, or about 20 days later (if it was a Jewish leap year). Every 19 years, the two dates will coincide (if there was a secular leap day - February 29 - the two may be one day apart). What is always true with the Jewish calendar is that festivals which occur on the 15th day of a Hebrew month will take place at full moon. We should also point out that, had the Jewish calendar only been based on the lunar year of 12 lunar months without the added leap months, the Jewish calendar would have been like the Muslim one, where there is a net difference of 11 days each year as compared to the solar year. Thus, the Muslim calendar shifts 11 days each year, and a given festival may occur in any season of the year. Ramadan, when Muslims fast throughout daylight for a full month, can be in the middle of the winter - but can also be in the middle of the summer, depending on the year.  

Question What is a day by Jewish law?

Answer According to Judaism, the day begins when night falls and continues until the following night. Evidence for this division can be seen from the very beginning of the Bible: "It was evening and it was morning - the first day." This is the time schedule followed for such matters as the observance of the Sabbath and the festivals. As it is unclear whether twilight is part of the previous day or of the following night, Jewish law requires a person to adopt the more stringent approach either way. This means that one must begin observing the Sabbath or festival before twilight (i.e., before sunset), but must observe it until the end of the following sunset (i.e., after nightfall).

Question What is the smallest unit of time in the Jewish calendar?

Answer While the Jewish calendar also divides the day into 24 hours, with 60 minutes to each hour, the minute is not divided into 60 seconds but into 18 halakim ("parts"), with each helek the equivalent of 3 aaa seconds. Practically speaking, this division is of almost no significance. In fact the only time it is used at all is in those synagogues which have the custom, on the Sabbath before Rosh Hodesh, of announcing the exact instant of the new moon. That time is always quoted in terms of hours, minutes and halakim.

Question What purpose do we serve when we pray? After all, does God need us to praise Him?

Answer This question implies a basic supposition - that one prays primarily because of the effect one's prayer has on God. Many Jewish thinkers, though, make it clear that prayer is meant to have an effect on man rather than on God. By praying, a person must contend with the fact that there is a Supreme Being above him. That awareness should cause him to improve his ways, and that, in turn, will affect God's relationship to the person. A great 19th century Jewish thinker, R' Samson Raphael Hirsch, noted that the word tefillah, the Hebrew word for prayer, is derived from the root, PLL, which implies judgment. Thus, when the person prays, he must, as it were, step aside and look at himself objectively, to see where he has come from and where he is headed.