סימן יט סעיף ה
מַתְחִילִין לוֹמַר טַל וּמָטָר, בִּתְפִלַּת עַרְבִית שֶׁל יוֹם שִׁשִּׁים לְאַחַר תְּקוּפַת תִּשְׁרֵי וְהוּא בְּיוֹם דּ' אוֹ בְּיוֹם ה' לְחֹדֶשׁ דֶּעצֶעמְבֶּער, וְאוֹמְרִים עַד פֶּסַח.
Introduction: The sixth berachah of Shemoneh Esrei begins, "Bareich aleinu …" It is a prayer for bountiful harvests and ample rainfall for the earth and its inhabitants. Therefore, when the season for rain arrives, we add a special prayer for "dew and rain" (tal umatar).
We begin to say tal umatar in the Shemoneh Esrei of Maariv of the sixtieth day after the Tishri (Fall) season,7The names of the four seasons (singular: tekufah; plural: tekufos) are: Tekufas Nisan (spring); Tekufas Tamuz (summer); Tekufas Tishri (fall); Tekufas Teiveis (winter). which comes on the fourth or the fifth of December,8In Eretz Yisrael the rain season begins earlier and tal umatar is said from Cheshvan 7.
The author’s dates were valid only until the year 1899. From 1900 until 2100 tal umatar is begun on December 5 during a regular year and on December 6 during a leap year. and it is said, until [the first day of] Pesach.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)