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.

"haftasonu" is incorrect it should be "hafta sonu"

BTW, It is pretty common to say "hafta sonunda". It is also common to say "hafta başında", "hafta içinde", "hafta ortasında".

Why would we say "sabahta", "akşamda" as long as we have the words "sabahleyin", "akşamleyin"? 

We don´t say pazarteside as it is grammatically incorrect. It should be pazartesinde. And pazartesinde is possible to say.

Thanks for your comment. I didn´t mean that "hafta sonunda" is not correct. But the people I know seem to prefer to say "hafta sonu" rather than "hafta sonunda" or

"Pazartesi günü" rather than "Pazartesinde".

Or am I not right?