2,807. "One Day" vs. "Today"

Hilchos Nedarim 10:1

If someone makes a vow or an oath not to taste food that day, he is only prohibited to do so until nightfall; if he said he wouldn’t taste food for one day, he is prohibited for 24 hours from the time of his vow. Therefore, even though he may eat after nightfall, one who vowed “today” shouldn’t do so without consulting a Torah scholar to absolve him of his vow. This was enacted out of concern that one might later make an oath not to eat for a whole day and come to eat after nightfall, because most people are not aware of the difference between the two scenarios.

Hilchos Nedarim 10:2

If someone vows not to taste food for “a day,” there is a doubt. One is therefore prohibited to eat for the whole day, the same as if one vowed for “one day,” though if he eats after nightfall, he is not liable to the penalty of lashes. If one vows not to taste food “this week,” he is prohibited for the rest of that week up to and including Shabbos, but he becomes permitted on Sunday. If he vows not taste food for “one week,” he is prohibited for a full seven days. If he vows for “a week,” there is a doubt. Accordingly, he is prohibited for a full seven days, but if he eats after Shabbos he is not liable to lashes, as we have said.