This site explains the procedure:
If day 31 is on Shabbat or a major Jewish holiday (other than
Chanukah, Purim or Chol Hamoed), the pidyon haben takes place on the
If day 31 is a public fast day, the pidyon haben is
held right before nightfall, and the festive meal is celebrated that
evening. (The wine that the kohen usually drinks during the ceremony
is drunk by a minor who is not yet obligated to fast.)
In summary, you should make the Pidyon itself, Sunday afternoon, probably after Mincha (i.e., between mincha & Ma'ariv). Daven Ma'ariv and then have the seudah.
You could also do the pidyon and seudah at night after Shabbat is done. On the same page it states:
Although the obligation to perform the ceremony technically begins at
nightfall following day 30 (since the Jewish day begins at nightfall),
the custom of Ashkenazic Jews is to hold the pidyon haben in the
afternoon of the 31st day
Note that if you are Ashkenazy (I made that assumption, hoping I'm correct) doing it in the afternoon is merely a custom, but, there's nothing halachically wrong with doing the pidyon at night.