In chutz la’aretz, outside Eretz Yisroel, we begin to recite the prayer for rain, “Ve’sein tal u’matar livrachah” in the brachah, blessing, of bareich aleinu in Shemoneh Esrei, the Amidah, sixty days after the tekufah of Tishrei (the Hebrew term for the autumn season), which is when rain is needed in large parts of the Diaspora. This year we begin on the evening of December 4th, and on a year preceding a (civil) leap year on the evening of December 5th. If one forgot to say ve’sein tal umatar, even if they only omitted the word umatar, they should say it before saying Hashem’s Name at the end of the brachah of bareich aleinu, or at other intervals throughout Shemoneh Esrei, as per the instructions in the siddur. If a person did not remember until they finished Shemoneh Esrei, they have to repeat the entire Amidah because of the importance of the prayer for rain. If it is already time for the next prayer, Shemoneh Esrei should be recited twice to make up for the omission. There is a general principle that for thirty days after a seasonal change in a prayer, if one is in doubt whether the correct version was said they must assume that they said the previous, incorrect one. However, after thirty days have elapsed, they can assume that they said the correct version.