Tashlich and Changes in the Davening
Tashlich
The practise of reciting Tashlich on Rosh Hashana is recorded in the writings of the Mahari”l. It is referenced in the Darkei Moshe and quoted by the Ram”o in Shulchan Aruch. The Ram”o writes that we go to the river and recite the Posuk “and cast into the depths all of our sins”.
The Magen Avraham references the Ariza”l that one should also say the Pasuk “Mi Keil Kamocha”. According to Kabbalah, this Pasuk records the 13 Attributes of Mercy. This practise is recorded in both Shulchan Aruch Harav and Mishna Berura.
The Ram”o does not write on which day Tashlich is said. However, the Acharonim, based on the writings of the Arizal, write that it should be said on the first day of Rosh Hashana after Mincha, before sunset.
When Rosh Hashanah falls on Shabbos
The Mateh Efraim writes that even when Rosh Hashanah falls on Shabbos, we should not change the Seder and Tashlich is still recited on the first day. This is based on the ruling of the Shevus Yaakov who infers it from the Mahari”l.
The Chida”h in Birkei Yosef writes that one should not recite Tashlich on Shabbos. Rather, they should recite it on the second day of Rosh Hashanah. He quotes this in the name of Rabbi Moshe Zacut.
However, Shaarei Teshuvah quotes the Chida”h in Machzik Bracha that the great Kabbalist Rabbi Yosef Irgaz would still recite Tashlich on the first day. After bringing the different opinions, Shaarei Teshuvah writes that our custom is like the Shevus Yaakov.
In a later work, Yosef Ometz, the Chida”h argues with the Shevus Yaakov, suggesting that he retracted his opinion. Ktzeh Hamateh suggests that this is because in a matter of doubt, it is best to not do anything. Other Poskim who write that Tashlich should still be recited on the first day include; Ben Ish Chai, Kaf Hachaim and Mateh Efraim.
Mateh Efraim warns that when performing Tashlich on Shabbos, one should be very careful to not carry anything outside of the Eiruv.
Amidah and Kiddush
One would not need to repeat the Amidah or Kiddush if one made a mistake, either on Shabbos or a weekday.
In the middle blessing of the Amidah, both in the text of the blessing and the conclusion, we have to mention Shabbos. The blessing concludes מקדש השבת ישראל ויום הזכרון. If one did not mention Shabbos either in the middle or the end of the blessing, they are required to repeat the Amidah. The same would apply to Kiddush.
If they mentioned Shabbos in the middle, but not in the end of the blessing, there is a Machlokes between the Poskim. In this case, due to the doubt, one would not go back or repeat the Amidah.
In the Amidah and Kiddush, the Acharonim write that we do not add the word באהבה prior to מקרא קודש as we would on one of the festivals. Presumably this is because Rosh Hashanah and Yom Kippur are days of Awe, not of love. Even though the Alter Rebbe brings this in the Shulchan Aruch, in the Siddur he includes the word באהבה on Shabbos.
The Mishna Berura quotes the Pri Megadim who writes that he has seen places that recite Tashlich on the second day when Rosh Hashanah falls on Shabbos. He suggests that perhaps this is because the rivers were situated outside of the city and there was a concern of people carrying Sefarim and the like on Shabbos.
Rav Ovadiah Yosef writes that in a place where the Tashlich location is within the Eiruv, Tashlich should be recited on the first day. Where Tashlich is performed outside of the Eiruv, it should be done on the second day.
The Sdeh Chemed writes that the custom is not to recite Tashlich on Shabbos.
What is the Chabad Custom?
In both the Shulchan Aruch and the Siddur, the Alter Rebbe does not write anything about Tashlich when the first day falls on Shabbos, suggesting that there should be no change and that Tashlich is still recited on the first day. It appears that this was the practise of the earlier Chabad Rebbeim.
The custom of the Frierdike Rebbe and the Rebbe Rashab was to perform Tashlich on the second day when the first day fell on Shabbos. The Rebbe suggests that the change in custom is because as a result of saying Tashlich on Shabbos, people were inadvertently coming to carry on Shabbos. This was the practise of the Rebbe.
Changes in the Davening
When Rosh Hashanah falls on a weekday, in both Kiddush and the Amidah, we say יום תרועה מקרא קודש - a day of sounding (the Shofar), a Holy convocation. However, when Rosh Hashanah falls on Shabbos, we substitute יום זכרון תרועה instead.
The reason for this is because when Rosh Hashanah falls on Shabbos, it is not a day of actually blowing the Shofar. Instead we rely on the remembering of the sounding of the Shofar, through the Pesukim of Shofaros. This distinction is brought in the Gemara to reconcile 2 different Pesukim, one which refers to Rosh Hashanah as יום תרועה and the other as זכרון תרועה.
Even though we do not sound the Shofar at night, we still change the Nusach because we are referring to the entire day.
