If you live in Israel, you need not read on.
December 4th is a significant day in the Jewish liturgical calendar. It is the day on which those who live outside Israel begin to add the words ותן טל ומטר - give us dew and rain. Here is the talmudic source for this change:
תענית דף י, א
משנה. בשלשה במרחשון שואלין את הגשמים. רבן גמליאל אומר: בשבעה בו, חמשה עשר יום אחר החג, כדי שיגיע אחרון שבישראל לנהר פרת
גמרא. אמר רבי אלעזר: הלכה כרבן גמליאל. תניא, חנניה אומר: ובגולה עד ששים בתקופה.
Mishnah: On the third of Marcheshvan we ask for rain [that is, we insert the words ותן טל ומטר into the Amidah prayer]. Rabban Gamliel said: it is started on the seventh.
Talmud: Said Rabbi Elazar, the law is like Rabban Gamliel. It was taught in a Beraita: Chaninah said: outside of Israel, it is inserted sixty days after the fall equinox.
This is the only Jewish ritual that is tied to a real solar event. That event is the autumnal equinox, the day on which the sun is directly overhead on the equator, and on which the lengths of day and night are (almost) equal.(Please do not write to tell me about Birkat Hachamah. It is not tied to any real solar or other event anywhere in the cosmos. See here for more details.)
This year the autumnal equinox was on September 22nd (at 9.54am EST to be precise). The Talmud tells us to begin saying ותן טלֹ sixty days later. So you take out the calendar and count. That will bring us to November 21st. But in every siddur with instructions you will read that we start to recite the addition at Maariv on the evening of December 4th. Which is tonight (or was last night if you are reading this in Australia). That’s a full seventy-three days after the equinox. How did that happen?
It is really not difficult to understand, and requires no skill in observational astronomy or non-Euclidian geometry. Here is the explanation in four easy steps.
The Julian Calendar
In the Julian calendar the solar year is exactly 365 1/4 days. That is, 365 days plus an additional 6 hours. This is also the length of the year according to the great talmudic sage of the second century, Samuel of Nehardea. And it is the assumed length of the solar year that is still used today in Jewish calculations. The actual length of the tropical year (i.e. one complete cycle of seasons) is slightly shorter: 365 days, 5 hours, 48 minutes and 46 seconds. That’s a difference of less than twelve minutes. Not much you say. And you’d be right, except that over a century that difference adds up to about 3/4 of a day. So every century the Julian calendar (365.25 days) falls behind the actual length of the year by three-quarters of a day.
2. The Gregorian Calendar
Over hundreds of years that difference adds up to many days, which became a real problem for the Church, when Easter was slipping further back in the calendar. The vernal equinox (a real solar event) had fallen on March 21st in the Julian calendar, but over the centuries had slipped back to March 11th. This threw the calculation of Easter into turmoil. To correct this (and other problems) Pope Gregory removed ten days from the calendar. By papal decree, the last day of the Julian calendar was on October 4, 1582. The next day became the first day of the Gregorian calendar, October 15, 1582. Now the spring equinox would fall on March 21, and everyone would be happy.
3. Jewish law stuck to the old Shmuel - Julian calendar
Rather than go along with this change, it was ignored by rabbis of the early modern period. They would rather have been wrong with Shmuel, than right with the Pope. They added back in the ten lost days. But by 1900 an additional three days of slippage had built up. The calendar of Shmuel is now some 13 missing days behind the Gregorian.
4. So add back in the missing days
If you add an additional thirteen days to the sixty days prescribed by the Talmud, you arrive at….December 4th! I told you it was easy. Here, try it for yourself:
Happy December 4th from Talmudology