The Mishna in the third perek of Masechet Nazir describes the process that a person must go through on the day he finishes his period of Nazirut. If this person had made a stam Nazirut (ie. an unspecified period) he must wait the minimum time period for a Nazir, that is 30 days. The Mishna states that following this, the Nazir must bring his korbanot and undergo the shaving procedure on Day 31. However, if this person brought them on Day 30 he would be yotze b’dieved.The reason for this is due to the concept of mikzat hayom kekulo (a partial day is equivalent to a full day).
This case of a stam Nazirus is distinct from a case where one does specify the time period of his Nazirut. If one was to make a specific vow to be a Nazir for 30 days, and shaves and brings the korbanot on Day 30, he does not absolve himself - even b’dieved. The Rambam is of the opinion that wherever one specifies a time period, for example 30 days, he means 30 full days - and cannot use this concept of mikzat hayom kekulo.
There is a girsain the Gemara (Nazir 5b) that explains this case where a person specifies the time period in a straightforward way. The Gemara states that when this person took on his Nazirut, he specifically mentioned in his vow that he was referring to 30 full days. The Rosh states that since he has mentioned full days in his vow, we do not allow him to use the leniency of mikzat hayom kekulo.
The Rosh also brings another version of the Gemara there which states that it is “as if he was referring to full days”. The Rosh explains that according to this version - we are stringent on a person where he specifies a 30 day time period. This is because had he not specified we would automatically allow him to use the partial day as a full day. However, since he mentioned a 30-day time period, which is superfluous, we act stringently with him and do not allow him to be absolved by fulfilling a partial day only.
There is a practical difference between the opinions of the Rambam and the Rosh. According to the Rambam whenever someone mentions a time period, they mean full days. The Rosh however holds that one must only count full days where the language they are using is superfluous. Therefore, in a case where one was to make a Nezirut vow for more than 30 days, (for example 100 days) and shave and bring korbanot on the last day (day 100) the Rambam and Rosh would be in disagreement as to the validity of this service. The Rosh would hold that since vowing to be a nazir for 100 days is not superfluous (as one specifies a time period that is different to an unspecified Nazir), we would hold mikzat hayom kekulo and the service is valid b’dieved. The Rambam however would hold that since a time period was specified, we do not use the concept of mikzat hayom kekulo, and the service would be invalid.
The Tosafot brings another reason for a difference in the law between someone who makes a stam Nezirus and one who vows to be a Nazir for 30 days. Tosafot say that when one makes a stam Nezirus he has intention to be bound by all laws of Nazir as they are written and explained in the Torah. In this case then we can use this law derived from the Torah - which is a partial day counts as a full day. However, one who makes a specific time period for his Nezirut is essentially ‘rejecting’ the Torah’s default Nazir period and is replacing it with his own - therefore we apply the laws of Nedarim which state that we go by “Lashon bnei Adam” - which means 30 full days.
The question is asked why mikzat hayom kekulo does not apply at the end of the term - but at the beginning of the Nazirut term - all agree that being a nazir for even part of the first day counts for a full day. Rabbeinu Peretz answers that there is a difference at the end of the term where the Nazir is looking to be absolved of a chiyuv. To explain further, there is no room for leniency and allow a partial day to count for a full day where one must complete a day to fulfill his Nazirut vow. However, at the beginning of the term, the reality of the situation is that one cannot do more than a portion of the day (i.e. if he made the neder halfway through the day), so mikzat hayom kekulo does apply.
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