When Shushan Purim falls on Shabbat, residents of Jerusalem spread the observances of Purim over three days: Megillah reading
& meshloach manot on Friday, Al HaNissim and Purim Torah reading on Shabbat, and the Seudah (meal) and mishloach manot on Sunday.
Why would the Seudah be postponed to a day which is not actually Purim anywhere, when one can no longer include Al HaNissim in birkat ha-mazon? On Shabbat I suppose there wouldn't be much to distinguish it from one's regular Shabbat meal? Wouldn't it make more sense to have the Seudah early-ish on Friday?