Student or Learner
1. What day will Tom come?
(it's talking about from Mon to Sun, which day will Tom come?)
2. When will Tom come?
(it's talking about what is the actual timing in particular day that Tom will come?)
Am I right?
***I'm not a teacher***
(1) asks specifically about the day while in (2) the answer is not that limited, so you could say: on Monday; at 4 pm; later in the afternoon etc.