Written by Rabbis Eli Gersten, Yaakov Luban, and Moshe Zywica of the Orthodox Union
The Rema (Shulchan Aruch OC 695:2) writes that when Purim is on erev Shabbat, it is preferable to begin the Purim seuda in the morning (before midday) so as not to detract from the honor of Shabbat.
The Mishnah Berurah (695:10) cites the Yad Efrayim who writes that if this will be difficult, one may l’chat’chila postpone the seuda up until three hours before Shabbat. Bedieved (after the fact), if one is unable to begin the seuda until later, one must still eat the seuda up until Shabbat. If one is still in the middle of the Purim seuda at shkia (sunset), when Shabbat begins, one must cover the food, recite Kiddush, and then continue the meal. The Mishnah Berurah writes that if this were to happen, one would recite Retzei in bentching, but not al hanissim. One cannot recite both retzei and al hanissim, since this would be a contradiction. Since we are required to recite retzei, this indicates that it is Shabbat and Purim is over. Therefore, one can no longer recite al hanissim.
