Here in the Diaspora, we begin reciting in the Amida the section of "Barech Alenu," in place of "Barechenu," on December 4th. The section of "Barech Alenu" contains the special prayer for rain ("Ve’ten Tal U’matar Li’bracha") and is therefore reserved for the rainy season. Jews in Eretz Yisrael begin reciting "Barech Alenu" earlier, on 7 Marheshvan.
If a person mistakenly recited "Barech Alenu" before December 4th, when he was to have recited "Barechenu," must he repeat the Amida? It could happen that when a person hears that the Jews in Israel began reciting "Barech Alenu" on 7 Marheshvan, he becomes confused and recites "Barech Alenu" himself, forgetting that in the Diaspora this change is made later. Must he repeat the Amida in this case?
The Radbaz (Rabbi David Ben Zimra, 1480-1574) addresses this question in his work of responsa (vol. 5) and rules that a person in such a situation does not repeat the Amida (listen to audio recording for precise citation). He explains that even though Diaspora Jewry begins reciting the special prayer for rain only on December 4th, rainfall before this date is not looked upon as a "Siman Kelala" (a sign of curse). The rainy season, after all, begins before December 4th, as evidenced by the fact that we begin reciting "Mashib Ha’ru’ah" already on Shemini Aseret. Therefore, one who mistakenly recites "Barech Alenu" before December 4th does not repeat the Amida.
The question remains, however, as to whether this ruling applies even before 7 Marheshvan. One might argue that the recitation of "Barech Alenu" does not invalidate the Amida before December 4th only because the Jews in Eretz Yisrael have already begun reciting this prayer. If, however, one made this mistake before 7 Marheshvan, when nobody has yet to begin reciting "Barech Alenu," perhaps he should be required to repeat the Amida.
In truth, Hacham Ovadia Yosef, in his work Halichot Olam (vol. 1, pp. 136-7), rules that one does not repeat the Amida if he mistakenly recited "Barech Alenu" even before the 7th of Marheshvan. He notes that in principle, the residents of Israel should begin praying for rain already on Shemini Aseret. This prayer was delayed until 7 Marheshvan out of consideration for the Babylonian Jews who would come to Jerusalem for Sukkot during the times of the Mikdash, and had to journey back to Babylonia after Shemini Aseret. The Jews in Israel did not wish for the Babylonian pilgrims to have to travel in the rain, and so they delayed the special prayer for rain until 7 Marheshvan. In principle, however, the time to begin reciting this prayer is immediately after Shemini Aseret. (In fact, the Ramban maintains that after the Temple’s destruction we should begin reciting "Barech Alenu" right after Shemini Aseret, though Halacha does not follow his opinion.) Certainly, then, if one mistakenly recited "Barech Alenu" even before 7 Marheshvan, he need not repeat the Amida.
Of course, this applies only if the mistake occurred after Shemini Aseret. If a person recited "Barech Alenu" during the summer months, when rainfall is not desirable, then he must certainly repeat the Amida.
Summary: Diaspora Jews begin reciting "Barech Alenu" in the Amida in place of "Barechenu" on December 4th. If one mistakenly recited "Barech Alenu" before this date, but after Shemini Aseret, he need not repeat the Amida. If this occurred before Shemini Aseret, or after Pesah, then he must repeat the Amida.
