The Jewish calendar seems complicated. Rosh Hashanah begins anytime from early September to early October. The new month (Rosh Chodesh) alternates between a one- day and two-day celebration. The year has 12 months, but sometimes 13. A new day begins at about 5 p.m. in December and not until after 9 p.m. in July.

In contrast, the secular calendar is simple and straightforward. The New Year always begins on January 1. Each new month begins with but a single day. Every year has twelve months, and each day begins at exactly 12 midnight.

Just when you thought you'd seen everything there is to see in the Jewish calendar, along comes the strange phenomenon of December 4 (or 5 or 6). In the daily Amidah, in the berakhah (blessing) for a good year, one is to insert a prayer for rain during the winter only. Winter is defined by the siddur as beginning with December 4 (or 5) and continuing until the onset of Pesach.

Doesn't it seem strange to base a Jewish observance on the secular calendar? And furthermore, what is the significance of December 4?

By the way, the prayer for rain (v'tein tal umatarb -- "give dew and rain") is not to be confused with the "mentioning of rain" -- mashiv haruach u'morid hageshem -- which begins on Shemini Atzeret and also continues until Pesach arrives.

The mishnah (200 C.E.) instructs us to begin saying the prayer for rain on the 7th of Marcheshvan, and this is the ongoing practice in Eretz Yisrael. Some believe that the reason for this date is that it allowed everyone who had traveled to Jerusalem for Sukkot two weeks to return home before the rainy season would begin.

In the Babylonian Talmud (Ta'anit 10a), the sage Samuel instructed that the practice in Babylonia was to delay the start of the prayer for rain until 60 days after the beginning of autumn, in order to wait until the conclusion of the harvest.

We follow the Babylonian practice. But 60 days after the beginning of autumn is November 22, not December 4. This is what happened. In the year 1582, Pope Gregory XIII "repaired" the former Julian Calendar, which had become 10 days off from the actual cycle of the Earth around the sun. Pope Gregory declared that the day after October 4, 1582, would be October 15, and furthermore, that there would be no leap year in 1700, 1800, 1900, and all subsequent century years whose first two digits do not form a number divisible by 4. In other words, 2000 was a leap year, but 2100 will not be.

The Jews of the Diaspora did not change with the Gregorian calendar. Rather, they continued to count exactly 365 1/4 days until beginning the prayer for rain.

So, in the year 1582, the Jews began to pray for rain, not on November 22, but on December 2. When there was no leap year in 1700, the day for the prayer moved forward one additional day to December 3. After 1900, it had moved to December 5. It will remain here until after 2100, when the prayer for rain will begin, for the next century, on December 6.

When we say December 5, we really mean we begin to say the prayer at the Ma'ariv (Evening) service on December 4. To make things even more confusing, the schedule changes slightly in a year such as this one (2003) which precedes the leap year. In such a year, we don't start saying the prayer until Ma'ariv on December 5! Next year, we will be back to Ma'ariv on December 4, with the next exception occurring in 2007.

Now that we know the date, think about something more interesting. For us, this prayer isn't about rain, as much as it's about our identifying with the Jewish community of ancient Babylonia. This was the first successful center of Judaism and Jewish learning outside of Israel. The way we begin the prayer for rain causes us to remember this invaluable community. Just as the Haggadah tells us to "personally" experience the Exodus, the prayer for rain puts each of us, ever so briefly, back into the world of Jewish Babylonia.