The Rama (Rav Moshe Isserles of Cracow, 1530-1572) writes (Yoreh De’a 305:10) that it is customary after a Pidyon Ha’ben to have a festive meal. The custom among many Ashkenazim is to begin the meal before the Pidyon Ha’ben, and then perform the Pidyon Ha’ben during the meal. Sephardic practice, however, is to first perform the Pidyon Ha’ben and only then begin the festive meal.
It is customary to have a meal regardless of whether the Pidyon Ha’ben is performed at the optimal time – the child’s 31st day – or later. Either way, this meal should be held, and it is considered a Se’udat Misva – a meal in which it is a Misva to participate. At least ten people should participate in this meal, in order to publicize the fact that the child was redeemed.
Hacham Ovadia Yosef, in Yabia Omer (vol. 1, Orah Haim 27), writes that one is not required to eat bread at this meal. However, it is preferable to include meat and wine in the meal. This is mentioned by the Maharash Mi’Lublin.
There is a popular saying that one who eats at a Pidyon Ha’ben is considered as though he observed eighty-four fasts. There is no source for such a tradition, and thus its validity is uncertain.
The Pidyon Ha’ben meal has the status of a "significant meal" with respect to the laws of prayer. This means that if the Pidyon Ha’ben is performed at night, Arbit should be recited before the meal is begun.
It is permissible to perform a Pidyon Ha’ben and eat the Pidyon Ha’ben meal during Hol Ha’mo’ed. Whereas weddings are not held on Hol Ha’mo’ed, due to the principle of "En Me’arbin Simha Be’simha," which forbids combining two joyous occasions, this does not apply to Pidyon Ha’ben. Unlike at a wedding, we do not add the phrase "She’ha’simha Bi’m’ono" to the introduction to Birkat Ha’mazon at a Pidyon Ha’ben, and this proves that Halacha does not regard Pidyon Ha’ben as a "Simha" (joyous occasion) in the formal sense. Therefore, it may be held on Hol Ha’mo’ed.
A Torah scholar is not required to interrupt his Torah learning schedule for the sake of participating in a Pidyon Ha’ben meal. This is mentioned by Hacham Ovadia Yosef, in Soba Semahot (p. 347).
Summary: It is customary to eat a festive meal after the performance of a Pidyon Ha’ben. According to Sephardic practice, the meal begins only after the Pidyon Ha’ben. At least ten people should participate, and it is preferable to include meat and wine. One is not required to eat bread at this meal. If the Pidyon Ha’ben is held at night, Arbit should be recited before the meal is begun.