The opening Mishnah on today’s daf (=page) teaches that if someone took an oath to perform a mitzvah or to neglect a mitzvah, the oath has no meaning and the person would not be held liable for a wrongful oath even if he did not fulfill what he said.
In his Commentary to the Mishnah, the Rambam explains that if someone swears to neglect to perform a mitzvah he will not be held liable because the passage in the Torah regarding such an oath limits it to situations where the person can choose le-hara o leheitiv – that the individual controls whether to perform that particular action or not (see Vayikra 5:4). When a person swears that he will not perform a mitzvah, since he cannot do so, the laws regulating this type of an oath will not apply. It should be noted, however, that the laws of shevu’at shav will apply, since he invoked God’s name in vain when taking an oath that he could not keep.
In the case where he swore to fulfill a mitzvah, again the laws of a wrongful oath will not apply, since he had no choice but to fulfill the mitzvah. In this case, however, he will not be held liable for a shevu’at shav, since a person is allowed to make use of oaths or vows in order to encourage himself to fulfill commandments.
The Mishnah presents this ruling in contrast with the opinion of Rabbi Yeudah ben Beteira who believes that the laws of wrongful oaths do apply to someone who swears to fulfill a mitzvah. His logic is that if a person will be held liable if he does not uphold a voluntary oath that he takes upon himself, how much more will he be responsible if he does not uphold an oath that is taken regarding a mitzvah that he is already sworn to fulfill, that is, the commitment taken by every Jewish person on Mount Sinai.