(Old article from Matzav.com)

In chutz la’aretz, we begin to say Vesein Tal Umatar during Shemonah Esrei in the bracha of Boreich Aleinu.

If one became aware after completing the bracha of Mevareich Hashanim that one omitted Vesein Tal Umatar, one should wait to insert it right before “Ki atah shomeiah” in Shema Koleinu.

If one has already completed the bracha of Shomeiah Tefillah, one may insert Vesein Tal Umatar before saying Retzei. If one has already started Retzei, one must return to the bracha of Boreich Aleinu, which is the proper place for Vesein Tal Umatar. If one already completed the Shemonah Esrei and stepped backward, one must repeat the entire Shemonah Esrei (Shulchan Aruch with Mishna Berurah 117:5. See also Bi’ur Halacha).

(It is advisable to repeat 101 times [and at the very least 90 times] “Ve’es kol minei sevu’asah letovah vesein tal umatar” so as to make the inclusion of Vesein Tal Umatar habitual and fluent, thus eliminating any future doubt as to whether one included Vesein Tal Umatar in Shemonah Esrei or not.

Feel free to print this out and hang it up or distribute it in shuls and yeshivos.

(The following article is from 2011 so some dates may not be correct for this year.)

Now some background explanation:

It is well known that in chutz la’aretz, we usually begin vesein tal umatar livracha at Maariv of December 4^th. This year, as mentioned, we do not begin saying it until Maariv of December 5^th, tonight. Why is this year different?

We share the following from Rabbi Dovid Heber, who offered this background information in his weekly column, Calendar Calculations, in Yated Ne’eman:

The Shulchan Aruch (Orach Chaim 117:1), based on the Gemara in Maseches Taanis (10a), says that, in chutz la’aretz, we begin reciting vesein tal umatar on the 60^th day of tekufas Tishrei. “Tekufos” are seasons. For vesein tal umatar, we use the calculation of the Amora Shmuel, who says that the solar year is 365 days and 6 hours long. Each season is one quarter of that or 91 days and 7.5 hours.

This number is also very close to the astronomical calculation used to set up the secular calendar. (The difference is about 11 minutes. The explanation and ramifications of this difference are beyond the scope of this article.)

Since the secular year repeats itself at the same interval as tekufos, it stands to reason that tekufos will fall out on the same secular date each year (or perhaps one day different).

In this century, tekufas Tishrei always occurs on October 7^th. If we count October 7^th as Day #1, then December 5^th is Day #60. [There are 25 remaining days in October and 30 in November, for a total of 55 days. Thus, the 5^th day of December is Day #60.] The halachic day of December 5^th begins the night before. Therefore, usually (in this century), vesein tal umatar begins December 4^th at Maariv, which is the beginning of Day #60.

Every four years, including this year, the tekufah of Tishrei occurs at 9 p.m. on October 7^th, which is halachically the next day. For example, this year it occurred on Friday night, leil Yom Kippur, at 9 p.m., October 7^th. Halachically, this was already the next day (Yom Kippur, October 8^th). So, the 60^th day is Tuesday, 10 Kislev, December 6^th. Halachically, this day begins the night before. Therefore, we begin vesein tal umatar on Monday night, December 5^th.

The “fourth year” is the secular year preceding a secular leap year. This includes 2011, 2015, 2019, 2023, etc. During these years,vesein tal umatar begins on December 5^th at Maariv. In other years, the tekufah occurs at some time before sunset on October 7^th – either at 3 a.m., 9 a.m., or 3 p.m. – so, as explained above, the 60^th day is December 5^th and vesein tal umatar begins atMaariv of December 4^th (unless, of course, it is Friday night).

{Shmiel Gellman-Matzav.com Newscenter}