All I know about Was für ein is that it is a type of pronoun (but I don't know what type exactly it is). I also don't know how it translates into English.
is a phrasal expression which can be used in a question to ask "what kind of". For instance:
This translates into:
You can also add a verb inside it (so that it is split up), like in:
Please note that you need to change "ein" into "einen" because the subject is "du" and you are asking information about an object (accusative).
Another usage is in affirmative sentences where is basically means "what a...".
In this case the speaker expresses the simple fact that he/she has had a peculiar day (in a bad or in a good sense).
A vocabulary reference is here: