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.