"My aunt always works on Friday" should also be accepted. This would be the normal way to say it in American English.
The Polish sentence uses plural piątki, so accepting the seemingly singular "on Friday" wouldn't really make sense IMO.
OK, seems right... added.
My aint always works "on a Friday" should also be accepted as it's a common way of expressing this.
I guess we could then use singular in Polish, "w piątek"...