· Since the festive Purim meal is a Rabbinic requirement, the obligation to recite the Al ha-Nissim prayer in the Grace after meals on Purim is more stringent than the similar requirement on Chanukah.
· If one forgot to add Al ha-Nissim to the Grace on Chanukah, is not required to repeat it since the meals on Chanukah are obligatory. On Purim, however, there is a Rabbinic obligation to eat at least one meal. Hence, if one forgot Al ha-Nissim, he would be required to repeat the Grace after Meals. However, if one said Al ha-Nissim at one meal on Purim, one is not required to repeat it if one forgot to recite it in the Grace after subsequent meals. Some Rabbinic authorities maintain that there is no requirement to repeat the Grace if one forgot Al ha-Nissim, since the essential part of the prayer is the drinking of wine, over which Al ha-Nissim is not recited.
In any event, when one reaches ha-Rachaman in the Grace after meals, he should add the Al ha-Nissim prayer. Though one began his festive Purim meal during the day, and it extended into the night, he still recites A1 ha-Nissim in the Grace, for our considerations based on when the meal began. Among Sephardic communities, however, it is customary to recite A1 ha-Nissim only by day.