Tashlich is usually performed on the first day of Rosh Hashanah. If the first day of Rosh Hashanah falls on Shabbat, Tashlich is done on the second day of Rosh Hashanah. It may be performed up until Hoshanah Rabba (the last day of Sukkot), as some communities are anyway accustomed, except on Shabbat.
Special verses are recited next to a body of water, such as a sea, river, stream, lake or pond, preferably one that has fish (though when no such body of water was available, some rabbis were known to do Tashlich next to a well, even one that dried up, or next to a bucket of water). Upon concluding the verses, the corners of one’s clothes are shaken out; for males, this is usually done with the corners of the tallit katan (tzitzit garment).
Though Tashlich is not mentioned in the Talmud, its earliest reference appears to be in the book of the Prophet Nehemiah (8:1) which states, “All the Jews gathered as one in the street that is in front of the gate of water.” This gathering is known to have taken place on Rosh Hashanah.
The goal of Tashlich is to cast both our sins and the Heavenly prosecutor (a.k.a. the Satan) into the Heavenly sea. And when we shake our clothes after the Tashlich prayer, this is a tangible act to achieve the spiritual goal of shaking sins from our soul.
In case you have not done so and live in Brooklyn, you can go to the bay area on Emmons Avenue to do so. May we shake off our sins from the past and be sealed in the Book of Life for a healthy sweet new year!