The Mishna on our daf teaches about a person who accepts nezirut upon himself twice. First he says, “When my son is born, I will become a nazir.” Then he says, “I accept upon myself to be a nazir for 100 days.” It is clear to the Mishna that he must keep the laws of nezirut immediately. The question that arises is what to do if his son is born in the course of the next 100 days. The Mishna says that if his son is born any time up until the seventieth day, lo hifsid kelum – he does not lose anything. If he is born after the seventieth day, soter shivim – he has lost seventy.
There are two basic approaches to the rule taught in the Mishna.
According to Rashi, if the son is born before the seventieth day, the man interrupts his 100-day nezirut and keeps the standard 30-day nezirut that he accepted upon himself in celebration of his son’s birth – the nezirut that he accepted first. Upon completion of those 30 days, he can return to the 100-day nezirut that he had begun earlier. Should the son be born later than the seventieth day, since no nezirut can last less than 30 days all of the previous days are lost and the man must begin his 100-day nezirut over again.
Tosafot and the Rosh offer a different approach. Should the son be born prior to the seventieth day, then the 30-day nezirut in honor of the son’s birth can be included within the 100 day nezirut, and the man can fulfill both of his obligations during that period; he will bring two sets of sacrifices at the end of that period. If the son was born after the seventieth day, then the father must begin a separate 30-day nezirut to honor his son, and when that is over, he should return to and complete his original nezirut.