Tashlikh

Tashlikh (Hebrew: תשליך‎‎‎ "cast off") is a customary Jewish atonement ritual performed during the High Holy Days.

The ritual is performed at a large, natural body of flowing water (e.g., river, lake, sea or ocean) on the afternoon of Rosh Hashanah, the Jewish New Year, although it may be performed until Hoshana Rabbah. The penitent recites a Biblical passage and, optionally, additional prayers. Optionally, if the ritual is performed not on the Sabbath, small pieces of bread are thrown into the water.

The name "Tashlikh" and the practice itself are derived from an allusion mentioned in the Biblical passage (Micah 7:18-20) recited at the ceremony: "You will cast all their sins into the depths of the sea."

Most Jewish sources trace a year of the custom back to Yaakov ben Moshe Levi Moelin (d. 1427 in Worms) in his Sefer Maharil. There, he explains the custom as a reminder of the binding of Isaac. He recounts a midrash about that event, according to which Satan threw himself across Abraham's path in the form of a deep stream, in an attempt to prevent Abraham from sacrificing Isaac on Moriah. Abraham and Isaac nevertheless plunged into the river up to their necks and prayed for divine aid, whereupon the river disappeared.

...
Wikipedia