the next day: the day immediately following a stated point in time
This word names one particular day, the one that comes right after a day already fixed in the story. It is not a free-floating "tomorrow" but a relative marker: the morning after the feast, the day after the battle, the next day following the sabbath. Because of that, it almost always shows up as a time-stamp inside a narrative, usually in a set phrase like "on the next day" or "the day after."
The underlying idea is simple and steady across the Hebrew Bible: whatever just happened, this points to what came the following day. Sometimes that is a happy sequel (Israel eats the produce of the land the day after the Passover), and sometimes a grim one (Saul's body is found the day after the fighting), but the meaning of the word itself does not change. It simply carries the reader forward by one day.
Because it is fixed to a reference point, it works differently from the ordinary word for "tomorrow." "Tomorrow" is measured from where the speaker stands now; this word is measured from an event the text has already described. That is why it reads so naturally as a storyteller's phrase, moving the account from one day to the next.