Source and decoding - Taanit 2a
מֵאֵימָתַי מַזְכִּירִין גְּבוּרוֹת גְּשָׁמִים? רַבִּי אֱלִיעֶזֶר אוֹמֵר: מִיּוֹם טוֹב הָרִאשׁוֹן שֶׁל חַג. רַבִּי יְהוֹשֻׁעַ אוֹמֵר: מִיּוֹם טוֹב הָאַחֲרוֹן שֶׁל חַג.
Rabbi Eliezer says: from the first festival day of Sukkot.
Rabbi Eliezer says: from the first festival day of Sukkot. Rabbi Yehoshua says: from Shemini Atzeret, the eighth day following the seven days of Sukkot. The dispute concerns mentioning rain, not requesting it: Rabbi Yehoshua argues that rain during Sukkot is a sign of a curse, while Rabbi Eliezer replies that he is not requesting rain now but mentioning God's power to bring it in its season.