In the diaspora, we begin to pray for rain in the Amidah 60 days after the start of tekufas Tishrei (the autumn season), because rain before then is bad for the crops in Babylonia. This is not a fixed date on the lunar Hebrew calendar since it involves solar seasons. It is the night of December 4. When the Gregorian calendar skips a leap year, every 400 years, the correct day moves one day later. Starting in 2100, it will be the night of December 5.
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