Q. Is the Seuda some shuls or private people make on the last day of Chanuka a Seudas Mitzva that one should attend or also do? Since it is not mentioned anywhere, why should one take any part on it?
A. The last day of Chanuka is called by many Zois (this is) Chanuka since it represents the highest point achieved in the process of adding lights on this most important festivity. (Pardes Eliezer 13, Zois Chanuka, Imrei Noam and many others)
It is indeed traditional in many Chassidic circles to celebrate the day of Zois Chanuka with a Seuda that begins at the end of the last day of Chanuka and often extends into the night. Likutei Mahariach mentions that the reason is due that whenever we perform a mitzva that involves a prolonged time, we celebrate when we reach the end, as we do with the Siyum of the learning of a tractate of the Talmud. Beis Aharon mentions that his father would sing on the occasion 'Hadran Aloch Chanuka' (Nitei Gavriel - Chanuka 56: 3)
Horav Shlomo Miller's Shlit'a opinion is that one should follow the traditions of his family and congregation.
See next question.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit'a