"He swims every Wednesday and Saturday."
Translation:Er schwimmt jeden Mittwoch und Samstag.
Duolingo has previously made the decision to stick with only the High German translation Samstag for "Saturday". You'll find that's generally the ruling on dialects (even major ones).
Except, the use Sonnabend is not a dialect. It's a variant more prevalent in Northern Germany, true, but it is High Standard German (unlike Sünnobend, Saterdag, Sünnavend).
"Sonnabend ist der zweite hochsprachliche Begriff für den Samstag, er steht im Nordosten..."
( http://www.zeit.de/2012/30/Deutschlandkarte-Samstag )
Sonnabend was used by my grandparents, in the middle of Germany, but in my generation or the one of my parents I did not hear it anymore.
From Hanover, the epitome of Standard High German.
I don't see that choice in the internal Translation Guidelines, and when I had a look at that unit, all the other (older) example sentences for Samstag all accepted Sonnabend in the reverse translation.
As far as I can tell, this was simply an oversight -- this sentence is newer and the contributor who added it didn't think to add the alternative Sonnabend in the reverse translations.