On the first day of Rosh Hashanah after the afternoon prayer, we go to a lake, river or sea (preferably a body of water that has fish), and recite the Tashlich prayers, wherein we symbolically cast our sins into the water and leave our old shortcomings behind us, thus starting the new year with a clean slate.

If the first day of Rosh Hashanah falls on Shabbat, Tashlich is done on the second day of Rosh Hashanah. If one is unable to perform this ceremony on Rosh Hashanah, one may do so until the last day of Sukkot (this year, October 11, 2017).

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Text of Tashlich (Hebrew)
Text of Tashlich (English)

Click here for more about Tashlich, including many reasons for this ancient ritual.