The laws concerning the mention of wind and rain are detailed in the Shulchan Aruch, which dictates that we begin reciting the phrase mentioning God as the One who makes the wind blow and the rain descend during the second blessing of the standing prayer. This practice commences during the additional service on the final festival day of the autumn season, known as Shemini Atzeret, and continues until the additional service on the first day of Pesach.
The Mishna Berura explains that this mention is placed specifically in the second blessing of the standing prayer because that blessing deals with the resurrection of the dead. Since rain sustains the world and brings life to all living things, its power is considered analogous to the resurrection of the dead. While it might seem appropriate to begin mentioning rain on the first day of Sukkot—given that the world is judged for its water supply at that time—we refrain from doing so because rain is considered a sign of a curse during that festival. If it rains, one cannot fulfill the commandment of dwelling in the festival booth; therefore, we wait until the seven days of dwelling in the booth have passed before mentioning rain in our prayers.
Logically, the mention of rain should begin as soon as the holiday of Shemini Atzeret starts during the evening service. However, the law establishes that we wait until the additional service the following morning. One reason for this delay is that not everyone attends the evening service at the synagogue. If the change were made at night, it would result in a lack of uniformity, with some people mentioning rain and others not, creating division within the community. The change is not made during the morning service because such a transition requires a formal public announcement by the cantor or the synagogue beadle. Such an announcement cannot be made immediately before the morning standing prayer because the law requires the blessing regarding redemption to be followed immediately by the standing prayer without any interruption. Consequently, the announcement is made before the additional service, ensuring the entire congregation is aware of the transition.
If an individual makes a mistake and mentions rain during the evening or morning services of Shemini Atzeret before the community has officially begun the practice, they do not need to repeat the prayer. The transition back to omitting the mention of rain occurs during the additional service on the first day of Pesach. Similar to the start of the season, this timing ensures communal consistency and is preceded by a public announcement. In the Sefardic tradition, the mention of rain is replaced by a mention of the descent of dew during the summer months. The announcement of the mention of dew serves as the official signal for the congregation to stop mentioning rain.
The Rama notes a distinction for those following Ashkenazic customs, where it is not the practice to mention dew during the summer. In such communities, a public announcement to simply "stop" mentioning rain is avoided, as it might appear as though the congregation is rejecting or disparaging God’s blessing of rain. Instead, the congregation continues to mention rain during the silent additional service. It is only after they hear the cantor omit the mention of rain during the public repetition of the additional service that they officially switch. Consequently, for these communities, the individual transition to omitting the mention of rain only takes effect starting from the afternoon service.
Regarding errors during the Pesach transition, if a person mistakenly omits the mention of rain during the evening, morning, or additional services of the first day of Pesach, they are not required to repeat the prayer. This is true for all customs, as the period for mentioning rain is technically concluded, and the delay in stopping is due only to communal regulations. Furthermore, in the Sefardic tradition, the inclusion of the mention of dew protects the validity of the prayer even if the specific mention of rain was omitted prematurely.
The Halacha Berura clarifies that the mention of rain in the blessing titled Atah Gibbor is categorized as praise and glorification of God rather than a personal request or petition. This is why it is situated within the first three blessings of the standing prayer, which are dedicated to divine praise. If a person mistakenly begins mentioning rain during the evening or morning services of Shemini Atzeret, the procedure for correction depends on when they realize the error. If they remember before reciting the concluding signature of the blessing, they should return to the beginning of that specific blessing. However, if they have already completed the blessing and recited the signature, they do not go back and repeat the prayer.