I wrote "Venez le samedi", what's wrong with that?
because for some odd reason samedi means 'on saturday' as well as the day's name!
Le samedi means every Saturday in this context.
I thought that the tu form of the imperative tense didn't have an s. Is this some kind of exception, or did I get that wrong?
The rule is right but it is limited to the verbs which end in -es in the indicative present for for "tu":
What are some examples of verbs that end in "-s" in the imperative mood besides "viens"?
Dors ! Apprends ! Mets ! Bois ! Pars ! Finis !
Why viens instead of venez?
Viens is the 2nd person singular of "venir", while venez is 2nd person plural - I believe either should be OK here!
venez le Samedi
If the sentence said "Come Saturday" then it would be "Viens Samedi" or "Venez Samedi".
The fact it specifies to come on Saturday it then should translate to 'Viens le Samedi" or "Venez le Samedi".
I don't get why Duo marks this wrong.
Because it is wrong. "Come Saturday" and "Come on Saturday" mean the same.
Days of the week are capitalized in English and German but not in French nor Spanish (don't know about Italian or Romansh)