1,120. When to Perform Pidyon HaBen

164:2 If the father told one kohein he would redeem his son from him, he may not change his mind. If he did change his mind and redeemed the baby from another kohein, the redemption is valid.

164:3 We don't redeem the first-born son until he is more that thirty days old. We redeem him immediately on the thirty-first day so as not to delay the mitzvah. We don't redeem on Shabbos or yom tov but we do on chol hamoed. The practice is to redeem by day but if the thirty-first day has passed and they have not redeemed the baby, or it was Shabbos, yom tov or a fast day, they should redeem him immediately the following evening rather than waiting for the next day and further delaying the mitzvah.