When I find out the day I will visit you, I LL write to you. Does it sound natural?
In an informal letter, does it sound ok?
How would you put it?

Thank you

"When I find out the day I will visit you, I'll write to you."  This sounds pretty natural and would work just fine.  

As a native (Midwest American) speaker, I might say "When I find out the day I visit ... ," leaving off the "will" before "visit."