2,762. Traveling Today, Eating Tomorrow
Hilchos Nedarim 4:15
Let’s say that someone declares produce to be prohibited tomorrow if he travels to a certain place today. In such a case, he may travel to that place today and the produce is prohibited to him tomorrow. The same is true in all comparable cases. This is because one is more careful about not violating a prohibition than he is about keeping a condition that results in something permitted being rendered prohibited.
Hilchos Nedarim 4:16
Let’s say that someone made a vow to fast for ten days of his choosing. He was fasting one day but he broke his fast for the purposes of a mitzvah or to honor an important person. In such a case, he may eat and make up the fast on another day. The reason for this is because he didn’t name specific days to fast when he first made the vow. However, if he made a vow to fast today, but he forgot and ate, he must resume the fast. If he made a vow to fast for a day or two and, after he started his fast, he forgot and ate, that day’s fast is forfeit and he must fast again.