"How was your day" is the most common form :) But you can use both forms :)

How did your day go is usually followed by something else like: ...at work. ...with the new teacher....something like that. It is meant to be used to refer to a situation and how it was for someone. It can also be used alone when both people know about the specific situation referred to. How was your day is much more general.

Both are fine. We tend to use 'How did your day go?' when you know there was something significant about the person's day, for example, if it was their first day in a new job. 'How was your day?' is a more general enquiry, but the difference is quite subtle.

I agree with Andi. I hear a lot of people saying "How was your day?" or "How was your weekend?"