The Shulchan Aruch (188:10) rules: “If one is eating a Shabbos meal and Shabbos goes out, recite Retzei should be said in Birchas Hamazon. The same is for Rosh Chodesh, Chanukah and Purim. If the meal continues into the night, Yaaleh v’yavo or Al HaNissim should be recited in Birchas Hamazon, as we go after when the meal started”. The same halachah appears in 695:3, however, there the Mechaber cites two opinions in regard to saying Al HaNissim for a Purim seudah that continued into the night, the Rema says the minhag is to say Al HaNissim.
However, when Purim falls on erev Shabbos, and the Purim seudah continues into the night (i.e. into Shabbos), things are a bit more complicated. The Mishnah Berurah (s.k. 15) writes that since it’s Shabbos, Retzei must be recited, and it’s contradictory to say both Retzei and Al HaNissim, as it’s either Purim or Shabbos, and since Retzei is more stringent than Al HaNissim (presumably because, if one misses out Retzei he must bentch again, whereas if one misses out Al HaNissim he doesn’t) he should recite just Retzei, as is clear from the Chayei Adam, and not go like Magen Avraham who suggests otherwise.
In light of the above, yesh lehistapek [there is room for doubt], what to do when seudas shlishis continues into the night of Purim: On the one hand Retzei should be recited as we go after the start of the meal. On the other hand, since it is now Purim, seemingly Al HaNissim should be recited (like the Mishnah Berurah says, that when Purim falls on Friday and the meal continues into Shabbos, one must recite Retzei as it’s now Shabbos) and one can’t say both as it’s contradictory. However, the Mishnah Berurah said that Retzei is more stringent, therefore, perhaps in our case as well, Retzei should be said and not Al HaNissim.
The truth is, the Mishnah Berurah discusses this very shailah. The Mishnah Berurah is talking about when Rosh Chodesh falls on motzei Shabbos, and seudas shlishis continued into the night, and one ate bread at night as well. Some say, Yaaleh v’yavo should be recited as everyone agrees that one is obligated to do so, and Retzei is a machlokes if it should be said after nacht, and since it is contradictory to both, just Yaaleh v’yavo should be recited. Some say, both Retzei and Yaaleh v’yavo should be recited, and we are not worried about the contradiction, as Retzei is recited because the meal started on Shabbos, and Yaaleh v’yavo is recited because of what was eaten at night. Then the Mishnah Berurah says: If, however, Chanukah and Purim fell on motzei Shabbos, even if the meal continued into the night only Retzei should be recited, as mentioning Chanukah and Purim in Birchas Hamazon is only ever optional, as is clear from the Mechaber (187:4).
