Learn French: À quelle heure est-ce que tu vas chez le médecin demain ? - At what time are you going to the doctor tomorrow?

Both ask about time, but Quand is used for a general 'when' (like what day or month). À quelle heure specifically means 'at what time' on the clock. Because a doctor's appointment happens at a specific time, À quelle heure is the most natural choice.

Why is there an À at the very beginning of the question?

In French, if the answer to a question will use a preposition like à (at), the question usually starts with it too. Because you will answer with something like 'at 10 o'clock', you ask 'At what time...?' (À quelle heure...).

How does est-ce que work when there are question words at the start?

est-ce que is a handy phrase that turns a statement into a question. When you use a question phrase like À quelle heure, you put that phrase first, follow it with est-ce que, and then add your normal subject and verb (tu vas).

Why do we say chez le médecin instead of au médecin for 'to the doctor'?

In French, you do not use à (to/at) when you are going to a person. Instead, you must use chez (at the place or office of) followed by the person or their profession. Going 'to the doctor' is always chez le médecin.