When

"Its redemption [shall be performed] from the age of a month"—Numbers 18:16.

This means that the pidyon haben (firstborn redemption ceremony) should be held on the thirty-first day after the boy's birth. (Both the day of birth and the day of the pidyon haben are included in the 31.) Although technically the obligation begins with the nightfall following day 30 – for the Jewish date begins with nightfall – it is customary to hold the pidyon haben in the afternoon of the 31^st day.1 Sephardic custom, however, is to schedule the pidyon haben for the first possible opportunity—the night preceding the 31st day.

• If day 31 is on Shabbat or a holiday (other than Chanukah, Purim and Chol Hamoed), the pidyon haben takes place on the next weekday.
• If day 31 is on Shabbat or a holiday, the pidyon haben takes place on the next weekdayIf day 31 is on a Friday, the meal should be scheduled for early in the afternoon, and only a minyan of men (in addition to the father, the kohen, and close relatives) should participate, so that the meal does not detract from the Shabbat festivities later that evening.
• 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 which the kohen usually drinks during the ceremony is drunk by a minor who is not yet obligated to fast.)
• If day 31 falls out on the day before Passover, the pidyon haben should be held in the morning, early enough so that bread can be eaten during the festive meal.

If, for whatever reason, the 31^st day has passed and the pidyon haben did not take place, the baby's father should do it at the earliest possible opportunity.

If the firstborn reaches the age of majority (Bar Mitzvah) without having had a pidyon haben, he is obligated to redeem himself. Click here for more on this topic.