The Sages instituted a special passage to be inserted into Birkat Ha-mazon on Shabbat: “Retzei Ve-haĥalitzenu” (“Favor and strengthen us”). In it we ask that our Shabbat rest and our fulfillment of Shabbat mitzvot find favor with God, and that God allow us to keep Shabbat with no sorrow or anguish. Since this is a petition, the Sages ordained that it be recited during the third berakha, which is also petitionary. In order to return us to the topic of this berakha, the added prayer ends with a request about Jerusalem and the redemption. We then return to the paragraph of “U-venei Yerushalayim.”
One who forgot to recite Retzei but remembered before beginning the next berakha (Ha-tov Ve-hametiv) must add the following prayer: “Blessed are You, Lord our God, King of the universe, Who lovingly gave Shabbatot for rest to His people Israel to serve as a sign and a covenant. Blessed are You, Lord, Who sanctifies Shabbat.” One then continues with Ha-tov Ve-hametiv. If he realized that he forgot Retzei after he had already begun Ha-tov Ve-hametiv, he must repeat Birkat Ha-mazon from the beginning. For the Sages required that Shabbat be mentioned in Birkat Ha-mazon. Thus if he did not mention Shabbat, he did not fulfill his obligation (SA 188:6).
All of this pertains to the first two Shabbat meals, when all agree that one is required to eat bread and therefore required to recite Birkat Ha-mazon. However, if one forgot to mention Shabbat in Birkat Ha-mazon at se’uda shlishit he does not repeat it, since be-di’avad that meal can consist of mezonot, so he is not obligated to recite Birkat Ha-mazon. Therefore, if he forgot Retzei, he does not repeat Birkat Ha-mazon. This is also the case regarding the insertion of Ya’aleh Ve-yavo on Rosh Ĥodesh or Ĥol Ha-mo’ed: Since one is not obligated to eat bread then, he does not repeat Birkat Ha-mazon (SA 188:8).
If one began to eat se’uda shlishit before shki’a and finished after tzeit, he says Retzei, since we follow the starting time of the meal. If Rosh Ĥodesh falls out on Saturday night, there is a serious doubt as to what he should mention in Birkat Ha-mazon. In order to avoid this situation, it is best to avoid eating bread after tzeit. That way it is certain that he should recite Retzei only.
. According to Ĥayei Adam 47:18, even if one began the berakha following Retzei and said “Blessed are you, O God, King of the universe” with the intention of continuing “Ha-tov Ve-hametiv,” he may correct himself and continue with “Who gave Shabbatot…” However, according to most Aĥaronim, even if he only said the first word of the berakha, he is already required to repeat Birkat Ha-mazon, since this shows that he has forgotten about the special Shabbat request. In practice, the ruling in BHL inclines toward stringency and requires that he repeat. Ben Ish Ĥai, Year 1, Ĥukat 20 states similarly. However, Kaf Ha- ĥayim 188:28 and Yabi’a Omer 6:28 side with Ĥayei Adam.
. However, according to Ben Ish Ĥai, Year 1, Ĥukat 20 and 22, if one recited Birkat Ha-mazon before tzeit and forgot Retzei, he must repeat Birkat Ha-mazon even at se’uda shlishit. This is because on a mystical plane there is no difference between se’uda shlishit and the rest of the Shabbat meals. In practice, I recorded the opinion of SA and most poskim that one does not repeat.
. If one continued eating bread after tzeit, then it would seem that he should recite Retzei based on when the meal began; in contrast, he should recite Ya’aleh Ve-yavo based on when the meal ended. Poskim disagree about what to do in this case: many feel that if one ate bread after tzeit, he must recite Ya’aleh Ve-yavo but not Retzei. Thus state MB 188:33 and Yaskil Avdi, OĤ 7:27. But according to Ben Ish Ĥai, Year 1, Ĥukat 22 and Yalkut Yosef 291:18, since he did not make havdala and he began the meal on Shabbat, he should recite Retzei and not Ya’aleh Ve-yavo. According to Taz 188:7, if he recites Birkat Ha-mazon after tzeit he should mention both Shabbat and Rosh Ĥodesh. This is also the opinion of Magen Giborim and Bigdei Yesha. (Therefore, le-khatĥila one should recite Birkat Ha-mazon before tzeit.)