Hafta sonude ne yapıyordun? What did you do on the weekend?

Thank you

 Hafta sonuda ne yaptın

My try ( but in English we tend to say AT the weekend)

 ..sonunda...

My try

I wanted to put the n there but I wasn`t sure , always go with your first instinct

I´d just say: 

Hafta sonu ne yapıyordun?

Yes but isn`t ne yapıyordun = what were you doing and ne yaptın= what did you do

 depending on the context the above phrase can be translated both "what did you do" and "what were you doing" /at least I think so)/

The da in ....hafta sonuda means in/on/at

 yes, generally -da/-de, -ta/-te means so.

but we do not say sabahta, akşamda or pazarteside  i.e. referring to some time period. the same thing applies to haftasonu

Unless you´re American.