A. Overview

The month of Heshvan is without festivals and is, for this reason, known as Marheshvan – a hint of the mundane, rather than the sad. However, in addition to meaning "bitter", the prefix also means "a drop of water".

While the Prayer for Rain is recited on Shemini Atzeret ("as soon as the lulav is laid down" – i.e. in Israel), when we are no longer commanded to dwell in the Succah, the regular Amidah prayers include only the phrase:

"Mashiv haruach umorid hagashem" – "Who causes the wind to blow and the rain to fall",
but not yet the actual prayer for rain to fall:

"Vetein tal umatar livracha" – "And grant dew and rain as a blessing". This is only said for the first time on the 7th of Heshvan (in Israel) and from the 4th/5th December (in the Diaspora).

The agricultural environment was and is part and parcel of all the Jewish Festivals. There are also specific celebrations and moments connected with water, and the blessing of rain, including: the Prayer for Dew on Pesach and its inclusion in the Amidah; Tashlich on Rosh Hashanah; Simchat Bet Hashoeva during Succot; the Prayer for Rain on Shemini Atzeret; the different date for its inclusion in the Amidah; Tevilah (ritual immersion).

In Biblical and Bet Hamikdash (Temple) times, two weeks therefore intervened between Shemini Atzeret before the prayer for rain to fall was pronounced, in order to allow all the pilgrims who had ascended to Jerusalem to return to their homes - and their largely agricultural livelihoods and ordinary lives – even as far away as the Fertile Crescent. This was also of spiritual significance – a return to the profane and a memory of the holy.

[The discussion in the Talmud demonstrates that the date was empirically deduced.]

Many commentators mention that the date of the prayer also coincides with the Parshah (weekly Torah reading) of Lech-Lecha, in which Abram is commanded to leave his homeland (the Fertile Crescent) in order to go to the Land, which is then promised to him and his descendents – an ascent in holiness - and that the prayer holds the same significance.

The Tal Umatar prayer was said throughout the millennia of Jewish Exile from Eretz Yisrael (the Land of Israel), but has returned to its full significance in modern Zionism and Israel. In the Diaspora, the first of which was in Babylon, where rain was not needed so early or so urgently, this prayer was inserted at a later date, in the evening of the sixtieth day after the autumn equinox – December 5th (6th preceding a leap year; 4th in the centuries further from the leap year correction century). The date was originally set according to the Julian (not Gregorian) calendar.

The only other Jewish prayer linked to the solar calendar is Birkat Hachamah (blessing the sun) in Nisan, once every 28 years.

B. Online References

Shema Yisrael
Presentation of the discussion from the Mishnah, Gemarah, and Halachah from the Shulchan Aruch; calculation of the date of the prayer in the Diaspora http://dafyomi.shemayisrael.co.il/taanis/insites/tn-dt-10.htm
Discussion of the Mishnah and Gemarah sources for the two different Prayers for Rain http://dafyomi.shemayisrael.co.il/taanis/insites/tn-dt-04.htm

Chabad-Lubavitch
Explanation of the law; insights into compassion for the traveler
http://www.sichosinenglish.org/books/sichos-in-english/28/17.htm
The difference between rain and dew |
http://www.sichosinenglish.org/books/sichos-in-english/50/28.htm#n234

Online Kitzur Shulchan Aruch
Related text in English [with print version] http://www.geocities.com/yona_n.geo/kizzur/kizzur19.html

Sephardic Institute
Detailed discusion of Halachic texts relating to the Diaspora date for saying Tal Umatar, the different calculations and schools of thought
http://www.judaic.org/halakhot/talumatar.pdf

Virtual Bet Midrash - Yeshivat Har Etzion
Discussion of the different dates for the prayer, including for the pilgrims themselves in Temple times (see last section of file)
http://www.vbm-torah.org/archive/salt-bereishit/03-5lekh.htm

Rabbi Eliezer Segal
Lively discussion of the origin of the Diaspora date for the prayer and the Solar calendar http://www.ucalgary.ca/~elsegal/Shokel/941215_Conundrums.html

Torah Org
Halachah Yomit on the dates of recital of additions, or accidental omission http://www.torah.org/learning/halacha/classes/class36.html
How it works against the solar or civil calendar
http://www.torah.org/advanced/haaros/out3-4.html

OU/NCSY
See: Note 17 on the Julian calendar dating of the prayer in the Diaspora
http://www.ou.org/ncsy/events/virtualshabbaton/luminari.htm