- Question: Why would you say both the paragraphs for Shabbos and Rosh Chodesh In benching at seuda shlishit if you bentched by night fall of a Shabbos where Rosh Chodesh starts that night but if Purim is on Friday and your Purim seuda runs into Shabbos then you only say the paragraph of Shabbos and not Purim in bentching?
A. The Mishna Berura in 188:33 says the reason you say both by a Shabbos leading into Rosh Chodesh is because the paragraph for Shabbos is going on the beginning of the meal then we say the paragraph of Rosh Chodesh (or Yom Tov) afterwards which goes on the second half of the meal. However according to that logic you should say both paragraphs when Purim goes into Shabbos?
B. There is a rabbinic concept called Tosefes Shabbos, adding on to the Shabbos before and after.
Answer: On Shabbos going into Rosh Chodesh there is actually additional minutes going into Rosh Chodesh so can say that when bentching it was for one half of the meal and then for the other half because Shabbos could have been pushed off longer to when they were ready to bentch and then it leads into Rosh Chodesh but once nightfall comes Purim is done and over, even if the meal started during the day so when bentching it is only Shabbos therefore only the paragraph of Shabbos is said. (See Dirshu Mishna Berura note 32 in 695:3:15.)