Usage advice: 什么时候vs哪个时候 I am writing a formal business email to a client, and at the end I would like to ask him when would be his most convenient time to have a phone call. The sentence goes like this:

请您让我知道哪个时候您最方便接电话。

I am not sure if it would be better to use 什么时候 instead.

If someone could please advice me on what subtle differences there are in the meanings of these two phrases, and suggest which to use, that would be very much appreciated!
I agree with zyy that 什么时候 is definitely better in the sentence.
哪个时候 is like literally translated from "which moment", but in Chinese, we express as "what time" so you should use "什么时候" or "什么时间"