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Birchas Hamazon after Purim Meal
In the grace after meals one adds Al Hanissim.
If one forgot to add Al Hanissim: If one forgot to say Al Hanissim and has already finished Bentching, he has nevertheless fulfilled his obligation and does not need to repeat Birchas Hamazon. If he remembered before reciting Hashem’s name in the blessing of “Al Haaretz Veal Hamazon”, even if he already said Baruch Ata, then he is to go back to Al Hanissim. If one has already recited Hashem’s name he is to continue Birchas Hamazon as usual without saying Al Hanissim, however in such a case, if one has not yet concluded Birchas Hamazon, then he is to recite “Harachaman Hu Yaaseh Lanu Nissim Kemo Sheasa Laavoseinu Bayamim Haheim Bezman…Bimeiy Mordechai”, in the orders of Harachamans which are recited. It is to be recited prior to the Harachaman of “Hu Yizakeinu”.
Does one recite Al Hanissim if the Purim meal extended into the night: If the meal extended into the night one is to nevertheless recite Al Hanissim in his Bentching. If one Davened Maariv prior to Bentching he no longer says Al Hanissim in Bentching. [Thus one is to avoid Davening Maariv prior to Bentching in order so he is able to Al Hanissim.]
The disguised visit of the Rama-Don’t forget to Bentch and Daven Maariv!
On Motzei Purim the Rama was accustomed to disguise himself in a mask and peculiar clothing, and walk around the city houses proclaiming “Don’t forget to Daven Maariv on time”.
 See Chapter 6 Halacha 4C for the full details of this subject and its relevant sources and footnotes!
 See Chapter 6 Halacha 4E for the full details of this subject and its relevant sources and footnotes!
 See Shulchan Menachem 3/323; Likkutei Sichos 39/282; Lev Haivri 1/4