"He swims every Wednesday and Saturday."
Translation:Er schwimmt jeden Mittwoch und Samstag.
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 )
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.