What is the basis for the common custom of continuing the Purim seuda into the night of shushan Purim? Is it because shushan Purim is also a yom tov? Is there some kind of kabbalistic reason for this custom? While the Rama mentions that one should try to eat the majority of the seuda by day, is it correct that one who only managed to eat a kezayis of bread before shkia and the rest of the seuda was eaten after shkia, nevertheless fulfilled the mitzvah?
According to the Rema O:CH 695-2 most of the seuda should be by day. There are however those that there is an inyan in kabalah that it should last into the night. See Piskei Teshuvos 695-5 who brings different sources for this.