Given how beloved the custom of Tashlich is, one would imagine the source for it to be the Talmud or early Rishonim. In fact, however, it is first mentioned in the classic 15th-century sefer of Western European minhagim authored by the Maharil. The Maharil records a custom to take a stroll after the Rosh Hashanah meal by a sea or river in order “to cast into the depths of the sea all our sins” (a paraphrase of Michah 7:19).
The Maharil explains that the custom of Tashlich – like many other Rosh Hashanah minhagim – is intended to evoke G-d’s mercy by recalling Akeidat Yitzchak. According to the Midrash, Satan tried to block Avraham en route to Har HaMoriyah by transforming himself into a river. Avraham, however, was not deterred, showing his dedication to G-d by wading through the water up to his neck in order to reach his destination. By walking near water on Rosh Hashanah, we allude to Avraham’s devotion.
The Eastern European counterpart to the Maharil – R. Isaac Tirna – has a different take on Tashlich (Sefer Ha’Minhagim, Rosh Hashanah). He claims that the symbolism of the custom is not the water but the fish living therein. They remind us that we are like fish that can be caught suddenly in a trap (in our case, the trap of sin). The Rema, in his Darkei Moshe commentary to the Tur (583:2), adds that fish are a good omen since the Evil Eye has no power over them and they reproduce abundantly.
It should be noted that none of these early authorities mention reciting anything as part of Tashlich; the custom was merely walking by a body of water (according to the Eastern European authorities, specifically one containing fish). Furthermore, this custom was originally known among Ashkenazim only.
Both these things begin to change under the influence of the Arizal (Sha’ar Ha’Kavanot, Sha’ar Ha’Shofar 5). His imprimatur on the Ashkenazic custom of Tashlich popularized it throughout the Jewish world. He also states that the custom is “even better” if the three concluding verses of Sefer Michah (which include the word “ve’tashlich”) are said. Additional, lengthy supplications appear in contemporary machzorim.
Although the earlier authorities note no specific time for Tashlich, the Arizal mentions the minhag taking place “on the first day of Rosh Hashanah, after Minchah, a bit before sunset” (this description clashes with that of the Maharil, who says Tashlich was done after the seudah). The Mateh Efraim (19th century) notes that it was not customary in his time to be particular about doing Tashlich before sunset; he also permits it to be done before Minchah if necessary (599:4, 7).
What happens if the first day of Rosh Hashanah falls on Shabbat (as it does this year)? The Pri Megadim (18th century) notes that he saw people in “some places” doing Tashlich on the second day of Rosh Hashanah when the first day fell on Shabbat (Eishel Avraham 587:5). He surmises that they didn’t do it on Shabbat because people generally carry things with them when they go to Tashlich (see also Birkei Yosef 583:6).
The Shvut Yaakov (18th century), however, forcefully maintains that one should do Tashlich on Shabbat and simply not carry anything. He rejects the idea that a person should avoid doing Tashlich on Shabbat lest he accidentally carry (Shu”t Shvut Yaakov 3:42). He further notes that the earliest source for Tashlich – the Maharil – clearly implies that Tashlich is done on Shabbat.
The Maharil notes that it is forbidden to feed wild fish on Yom Tov; he therefore exhorts people not to throw food into the water during Tashlich, or even to carry food for this purpose outside an area with an eruv since unnecessary carrying is prohibited on Yom Tov. The Maharil concludes: “And all the more so, this shouldn’t be done when Rosh Hashanah falls on Shabbat.” Such a warning would only have been necessary if people, in fact, were doing Tashlich on Shabbat.
Centuries ago, doing Tashlich on Shabbat was fairly uncomplicated. All one had to do was walk along a body of water. Even when the recitation of three verses became part of the minhag, doing Tashlich was not difficult. These could be said with relative ease from memory. Nowadays, however, people need a machzor to be able to read the several paragraphs that are now part of Tashlich, and carrying a machzor in the absence of an eruv is obviously forbidden on Shabbat. So today, there truly is room to be concerned about people accidentally carrying if they were to do Tashlich on the first day of Rosh Hashanah.
Rav Ovadia Yosef, however, permits doing Tashlich on Shabbat if one lives in an area with an eruv and takes care not to leave that area (Responsa Yechaveh Da’at 1:56). It also seems permissible to do Tashlich on Shabbat if one does not recite the prayers that were added later. Nevertheless, the common custom is to wait until the second day to do Tashlich.
Rabbi Yaakov Hoffman leads Washington Heights Congregation (“the Bridge Shul”). He has semicha Yadin Yadin from RIETS and is a certified Sofer. He can be reached at email@example.com.
 See Kitzur Shulchan Aruch (129:29) for yet another understanding of this custom.
 The custom further developed in some circles to include a symbolic shaking out of one’s garments. See Kaf Hachaim (583:30).
 It makes more sense to do Tashlich on the first day of Rosh Hashanah rather than the second because the first day is the main day of divine judgment. Thus, individuals who do Tashlich on Shabbat (taking precautions against carrying) should not be rebuked. However, since Tashlich is only a custom, one need not be excessively exacting about when one does it. Indeed, if one didn’t manage to do Tashlich on Rosh Hashanah, it can be done until Yom Kippur, according to some authorities (see Hilchot Chag Be’Chag, ch. 6 n. 84).