2,663. An Oath Not to Eat Matzah All Year

Hilchos Shevuos 5:18

If a person takes an oath not to eat matzah for a year or two, he’s not allowed to eat matzah on the nights of the Passover Seder. If he eats matzah, he’s liable for violating an oath of expression. In this case he hasn’t made a vain oath since his oath wasn’t to refrain from matzah on Seder night per se. Rather, he made an oath about times when eating matzah is optional along with times when doing so is a mitzvah. Since the oath is effective for those other days, it is also effective vis-à-vis Passover. The same is true in all comparable cases, such as if one took an oath never to sit in the shade of a succah or not to put a garment on for a year or two (which would preclude wearing tzitzis).

Hilchos Shevuos 5:19

Let’s say that someone took an oath that he put on tefillin or wore tzitzis that day when he had not in fact done so. In such a case, he is taking an oath of expression about the past because he’s describing something that (allegedly) occurred. Such a person isn’t taking an oath as to whether or not to perform a mitzvah.