"For every firstborn of the Children of Israel became mine, of man and livestock; on the day I struck every firstborn in the land of Egypt I sanctified them for myself. I took the Levites in place of every firstborn among the Children of Israel." (Numbers 8:17-18)
"...but you shall surely redeem the firstborn of man...Those that are to be redeemed - from one moonth shall you redeem according to the valuation, five silver shekels by the sacred shekel; it is twenty gera." (Numbers 18:15-16)
G-d originally designated the firstborn as those who would carry out the priestly duty in the Mishkan (Sanctuary). Shortly after receiving the Torah at Mount Sinai, the Jewish people collectively committed idolatry by serving the Golden Calf. The firstborns lost their sacred privilege and became unworthy of performing the priestly functions.
There were two segments of the Jewish people who did not take part in the worship of the golden calf; the entire tribe of Levi and the women.
When the Mishkan was completed, G-d commanded that the Levites replace the firstborns. At that time, there were 22,273 firstborn Israelites and only 22,000 Levites. Those Israelite firstborn men for whom a Levite could not be found to "exchange" with, had to redeem themselves by giving 5 silver coins to a Cohen (also from the Levite family).
Today, all Israelite male firstborns are redeemed in such a manner.
Who Needs to be Redeemed?
A father is obligated to redeem his son who is a firstborn to his mother. If the mother is the daughter of a Levite or a Cohen, or the father is a Levite or a Cohen,a 'redemption' is not needed.
If the mother is a daughter of a Cohen but married a non-Jew, the son still needs to be redeemed.
A child born through a cesarean birth does not need to be 'redeemed'.
If a daughter is born first, then no redemption of a subsequent male child is necessary.
Regarding a firstborn son that comes subsequent to a miscarriage or an abortion, a competent rabbi should be consulted.
Who and When?
According to the Babylonian Talmud the obligation to redeem the son falls mainly upon the father and according to the Jerusalemite Talmud, the mitzvah falls mainly upon the son; this difference has practical application in cases where the father neglects to redeem his son. (Likutei Sichos, Vol II, p. 42).
The obligation to redeem the firstborn son is the father's. If the father fails to do so or he cannot, the Beth Din (Jewish Court) does it.
A firstborn male who reached adulthood and has not been redeemed is obligated to redeem himself.
A child is not considered "established" until the thirtieth day after his birth. Therefore, a Pidyon Haben should take place on the thirty-first day. If it was done before that time, he is still not redeemed.
One should not wait beyond the thirty-first day. If the father waits beyond that time, he has transgressed a positive mitzvah for each day of delay.
It is preferable to have the Pidyon Haben during the daytime. However, if the thirty-first day after the birth has passed, the Pidyon should be made as soon as possible, even at nighttime.
One does not redeem his son on Shabbos or Yom Tov. During Chol Hamoed (the intermediary days of Yom Tov) the Pidyon can take place.
How Much?
The Torah says the child can be redeemded for five Selaim. Today it is acceptable to use five US silver dollars. This amount may be given to the Cohen in silver or the equivalent thereof (except land, servants, check, paper currency or a promissory note of debt owed to the father). If he redeemed with any of these, the redemption is deemed invalid.
Other Customs:
The act of Pidyon Haben is one that inspires G-d's blessing to all who participate in the celebration of the mitzvah. It is for this reason that one shares food from the Pidyon Haben meal with others. Some choose to do this with cloves of garlic and pieces of sugar as both have the ability of giving taste to more food, thereby enabling many others to participate in the mitzvah and its blessing.
The women are not forgotten. It was they who abstained from the golden calf. They would not even allow their husbands to take the slightest amount of their jewelry for sinful purpose. They take an active part in the Pidyon Haben by adorning the baby on his way to the Cohen with their golden jewelry. (This jewelry is returned after the ceremony).
Our sages have taught that partaking in the meal of a Pidyon Haben serves as a substitute for eighty-four voluntary fast days of penance. This is alluded to in the fact that the first two letters of the word "pidyon" have the gematria (numerical value) of 84. This can also be seen by reading into a rare word in the Torah found only when the Torah discusses Pidyon Haben. The Torah writes when talking about a pidyoN "the money of the pidyoM" (Numbers 3:49). This word can be read in Hebrew as Peh Daled Yom - 84 Days.
The mitzvah is unique in that it takes place in the course of the celebratory meal, not before it.
The mitzvah qualifies for the blessing Shehecheyanu to be recited - at a bris (according to Ashkenazi custom) Shehecheyanu is not said.
The meal of Pidyon Haben is classified as a "Seudas Mitzvah" - mitzvah meal.
Procedure at a Pidyon Haben:
The father brings the firstborn before the Cohen and the five Selaim of silver or its equivalent and informs him that the child is a firstborn, the first issue of the womb of his Israelite mother and says to him: "My Israelite wife has borne me this firstborn son."
The Cohen asks him: "Which would you rather have - your firstborn son or the five selaim which you are obligated to give me for the redemption of this your firstborn son?"
The father replies: "I want this, my firstborn son and here you have five sela'im which are required of me for the Redemption." As the father gives the Cohen the redemption money, the father says: "Blessed are You, L-rd our G-d, King of the Universe who has sanctified us with His commandments and commanded us concerning the redemption of a son. " And then he says: "Blessed are You, L-rd our G-d, King of the Universe, who has granted us life, sustained us and enabled us to reach this occasion."
Some observe the custom of preparing a festive meal in honor of the redemption of the firstborn. If there is wine, the Redemption ceremony is performed during the meal after the blessing over bread. The Cohen recites a blessing over wine immediately after the Redemption.
If there is no wine, the Redemption is performed before the meal and the Cohen recites a blessing over another beverage.