"Hun arbejdede ikke om fredagen."

Translation: She did not work on Fridays.

[VonnieStef]

Fredagen translates as the Friday so I was marked as wrong. Why not write "Hun arbejdede ikke om fredag".

[HeringDK]

That would not be right. It should be the Friday as you write (one incident) or på fredage (if general).
This exercise should be reviewed.

[RylieMcdon]

I was about to ask as well.

[Seraph259]

I'd also like to know this

[ndrwbrrw]

Quite.

[JROLD1]

Native english speaker here...It's a tricky sentence. I would say "She doesn't work on fridays" or "She's never worked Fridays".