Starting Thursday evening, the text of the Amidah standing prayer on weekdays in Israel changes to add the words for dew and rain in asking Heaven for agricultural bounty. For Sephardic Jews a different prayer is inserted, similar to the one Ashkenazim say when there has been no rain for a prolonged amount of time during the rainy season.
Communities outside Israel will follow suit on the evening of December 4th. The change will remain in effect until the beginning of Passover.