When Purim falls out on Friday, it is customary, le-khatĥila, to begin the meal before the afternoon, in honor of Shabbat. One who was not able to begin the meal before the afternoon should try to begin the meal at least three hours before shki’a. Be-di’avad, however, one may begin eating any time before shki’a. Either way, if one begins the meal close to Shabbat, he should try to limit what he eats, in order to eat the Friday night meal with a good appetite.
Alternatively, there is a custom to combine the Purim meal with the first Shabbat meal on Friday night. Some great Torah authorities follow this custom, while others recommend doing so only be-di’avad. In order to follow this custom, one must pray Minĥa and then begin the meal while it is still Purim. Then, around a half-hour before shki’a, one should accept Shabbat by lighting the Shabbat candles, place a covering over the bread, and recite kiddush over wine. Since one has already recited the berakha over wine (Ha-gafen) during the Purim meal, one should omit that berakha in kiddush. After kiddush, one continues the meal, making sure to eat a keveitza of bread, or at least a kezayit, for the Shabbat meal. At the end of the meal, one recites Retzei Ve-haĥalitzenu in Birkat Ha-mazon, adding Al Ha-nisim in the Ha-Raĥaman (“May the Merciful One”) section at the end of the prayer. After the meal, one prays Ma’ariv.
The source of the custom of combining the meals (called “pores mapa u-mekadesh,” literally, “place a cover and recite kiddush,” after a key element of the practice) is Pesaĥim 100a, and it is explained in sa 271:4. Me’iri followed this practice, le-khatĥila, when Purim would fall out on Friday (Me’iri, Ketubot 7a). Some Aĥaronim followed this practice as well. Kaf Ha-ĥayim 271:22 states in the name of Arizal that, le-khatĥila, this should not be done, because one must make kiddush after praying Ma’ariv. According to Me’iri and ma 695:9, one should recite both Retzei and Al Ha-nisim at their regular places. However, mb 695:15 states, based on Ĥayei Adam, that one should not recite Al Ha-nisim. I suggested above that one should recite Al Ha-nisim in the Ha-raĥaman section because it does not affect one’s fulfillment of the mitzva of Birkat Ha-mazon if it is recited there.