I have not decided to drop by you in which summer.
Is it right?

I have not decided which summer I will drop by you in....how about this??

Question

I have not decided to drop by you in which summer.
Is it right?

I have not decided which summer I will drop by you in....how about this??

Answer

Not correct.  
We say:
I have not decided which summer to visit you.
I have not decided which summer to drop by.

Answer

A more proper way to say that would be "I have not decided which summer to drop by" or "I have not decided which summer to drop by to visit you"