Sources: all collections; answered in English in the Scholarly style.

What Tashlich Is

The core text: Micah 7:19 — "וְתַשְׁלִיךְ בִמְצֻלוֹת יָם" ("and You will cast into the depths of the sea [all their sins]") [Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.11]. The surrounding verses (Micah 7:18–20) form the biblical backbone of the rite, including "נֹשֵׂא עָוֹן" ("Forbearer of iniquity") [Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.4] and "חֶסֶד לְאַבְרָהָם" ("kindness to Avraham") [Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.14].

[Rabbi Sacks on Yom Kippur Mahzor, Mincha, Haftara.7] notes that these three Micah verses are "usually also said on Rosh HaShana at Tashlikh, the 'casting' ceremony," and that they also appear at the end of the Yom Kippur Haftarah, creating intertextual resonance with Jonah.

The same verb "תַּשְׁלִיךְ חֲטָאֵינוּ" ("cast away our sins") appears as a petition in liturgical poetry: in Yaaleh (Yom Kippur Maariv) — "תַּשְׁלִיךְ חֲטָאֵינוּ וְתָחוֹן מַעֲשֶׂיךָ" [Machzor Yom Kippur Ashkenaz, Maariv, Yaaleh.18; Machzor Yom Kippur Sefard, Maariv, Yaaleh.18] — and in Selichot for Erev Rosh Hashanah — "תַּשְׁלִיךְ בִּמְצוּלוֹת־יָם כָּל־חַטֹּאתָם" [Selichot Nusach Polin, Erev Rosh Hashana.5:13; Selichot Nusach Ashkenaz Lita, Erev Rosh Hashana.7:13].

When and Where It Is Performed

Standard practice: - [Machzor Rosh Hashanah Edot HaMizrach, Tashlich.1]: After Mincha on the first day of Rosh Hashanah — even if it falls on Shabbat — one goes to the sea, river, spring, or well and recites Tashlich. Going outside the city is preferable. The presence of fish in the water is also preferable. The service opens with a passage from the Zohar. - [Ceremony and Celebration Family Edition, Rosh HaShana, Top Ten Facts.9]: The first afternoon of Rosh Hashanah; if it falls on Shabbat, moved to the second day. Involves "symbolically casting away our sins at a body of fresh water." Some throw breadcrumbs.

If the first day is missed: - [Machzor Rosh Hashanah Edot HaMizrach, Tashlich.1]: Recite on the second day after Musaf. - [Mateh Efrayim 601:8]: Tashlich on the second day is only done if it was impossible on the first day. However, if the first day fell on Shabbat, some have the custom to do it on the second day regardless (as detailed in siman 592).

When no body of water is accessible: - [Peninei Halakhah, Days of Awe, 3:15:6]: One recites Tashlich next to a water cistern or mikveh. If the sea or river is visible from a distance, some had the custom to recite it that way.

Origin and Obligation

- [Peninei Halakhah, Days of Awe, 3:15:3]: The custom originated among the Rishonim in Ashkenaz and spread over the generations to Sephardim, especially after the Ari z"l praised it. It is not obligatory. Several major authorities did not practice it: the Vilna Gaon (Gra), R. Chaim of Volozhin, and most Yemenite Jews. Some Hasidim perform it on a weekday after Rosh Hashanah. - [Torat HaOlah, Index 4:56]: Lists Tashlich as a topic treated in that work — "the reason for the custom of going to the water on Rosh Hashanah to perform Tashlich, and the reason for the verse 'תַּשְׁלִיךְ בִּמְצוּלוֹת יָם כָּל חַטֹּאתֵינוּ'" — indicating it was already a subject of theological explanation in the early modern period.

Who Did Not Practice It

- [Maaseh Rav 209:1]: "He [the Vilna Gaon] did not go to the river or well to recite Tashlich." This is the primary halakhic record of the Gra's non-practice, consistent with [Peninei Halakhah, Days of Awe, 3:15:3].

References

1. Machzor Rosh Hashanah Edot HaMizrach, Tashlich.1
2. Mateh Efrayim, .601:8
3. Peninei Halakhah, Days of Awe, .3:15:3
4. Sermons Unto My People, The High Holidays.14:1
5. Sermons Unto My People, The High Holidays.14:22
6. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.6
7. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.11
8. Maaseh Rav.209:1
9. Selichot Nusach Polin, Erev Rosh Hashana.5:13
10. Selichot Nusach Ashkenaz Lita, Erev Rosh Hashana.7:13
11. Peninei Halakhah, Days of Awe, .3:15:6
12. Torat HaOlah, Index.4:56
13. Rabbi Sacks on Yom Kippur Mahzor, Mincha, Haftara; The Book of Jonah.7
14. Ceremony and Celebration Family Edition, The Hagim, Rosh HaShana, Top Ten Rosh HaShana Facts.9
15. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.46
16. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.74
17. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.107
18. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.4
19. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.14
20. Machzor Rosh Hashanah Ashkenaz Linear, Tashlich.10
21. Selichot Nusach Lita Linear, Erev Rosh Hashana.7:76
22. Machzor Yom Kippur Ashkenaz Linear, Maariv Service for Yom Kippur Eve, Yaaleh.1163
23. The Koren Yom Kippur Mahzor; Ashkenaz, Additional Piyutim.53
24. Machzor Yom Kippur Ashkenaz, Maariv Service for Yom Kippur Eve, Yaaleh.18
25. Machzor Yom Kippur Sefard, Maariv Service for Yom Kippur Eve, Yaaleh.18