On Hoshanah Rabbah someone was eating in the sukkah and wanted to continue his seudah when the shmini etzeres started. He was pores mappah and continued. When he comes to bentch what should he say in Yaleh V’Yavo, Sukkah or Shmini Etzeres?
This question depends on the dispute among poskim concerning a Shabbos meal that enters Rosh Chodesh.
According to the Taz, both Retzei and Yaaleh Ve’yavo; according to the Bach one says Retzei alone; according to the Magen Avraham one says Yaaleh Ve’yavo alone (see Mishnah Berurah 188:33).
The Peri Megadim adds that according to the Taz, the same halachah will apply in a meal that crosses over from Hoshanah Rabbah to Shemini Atzeres: One has to say both.
It appears that this is the principle halachah: One must mention both Sukkos and Shemini Atzeres, for otherwise it is possible that one is not yotzei.
However, as the Mishnah Berurah writes, one should avoid the situation, and the same ruling will apply to the case of Hoshanah Rabbah – one should avoid the situation and conclude the meal before the entry of Shemini Atzeres.