Hi, Randy! Great question. I'll ask Mauricio what he thinks when he gets back, but this is the distinction as I understand it:
The question "¿Cuándo?" means "When?" Use it when you want to know in general when something will be. Answers could be, "Tomorrow," "September 15th," "Wednesday," "Very soon," "in the morning," et cetera.
But if you want to know specifically what time something is occuring, use "¿A qué hora?" which means, "At what time?" Your answer will always be a time, such as 5:00, 2:30, et cetera.
Think of it this way. If I asked, "¿Cuándo es el concierto? When is the concert," someone might answer me, "Tomorrow," or, "October 10." If I asked, "¿A qué hora es el concierto?" someone might answer, "At five o'clock."
So, to see if you understand, which would you use to ask:
1. When someone's birthday is.
2. When the train is leaving.
3. When the college semester starts.
4. When we're going to go to the movies.
The answers are:
1. ¿Cuándo? You want to know the DATE of birth, not time.
2. ¿A qué hora? You want to know the specific TIME the train leaves.
3. ¿Cuándo? You want to know the DAY that the semester starts.
4. Ooo, this is tricky. Usually, the question will be framed as, "¿Cuándo vamos? When are we going?" And the answer will be something like, "Really soon," or, "In a couple of hours." If you asked, "¿A qué hora vamos?" you'd be asking for the specific time that you'd all be planning to go. Chances are that the people you're going with never really thought about going at a specific time, more like whenever they got around to going, so the question would sound rather strange!
Isn't Spanish lovely?