How was your day?

This simple question always puzzled me.
Clear translation:
Kak byl tvoi den'?
or
Kak byl tvoim dnem?

Of course I understood that the best way to pose the question:
Kak provel tvoi den'?

And the answer:
U menya byl ochen trudnyi den'
or
U menya byl ochen trudnym dnem?

As you understand the question is about the use of Tvoritel'nyi padezh.

No instrumental case here.
«Kak byl tvoim dnem?» and «U menya byl ochen trudnym dnem?» are impossible.

«Kak byl tvoi den'?» and «Kak provel tvoi den'?» sound weird.
It would be better to say: «Как прошёл день?»

«U menya byl ochen trudnyi den'» is OK.