Common practice for a Shabbos bris is to make the celebratory meal on Shabbos.
Apparently, however, this approach was not always universal. The Magen Avraham (131:11) quotes from the Hagahos Minhagim that there were places in which the custom was that the seudah for a Shabbos bris was postponed until after Shabbos. Presumably the reason for the above is that a seudas bris on Shabbos would not be apparent that the meal was in celebration of the bris; therefore, they made a special meal on motzei Shabbos in honor of the bris. This can be compared to the accepted practice today that when Purim falls on Shabbos (which happens in our calendar only in Yerusholayim and other walled cities) the seudah is postponed to Sunday, in order to assure that the special Purim meal be noticeable.
