Sorry for the confusion with "Standard" rate.
I wanted to write the lowest rate for a standard room on weekdays (not during fairs and also not during very low times / just an average low rate available from the Starwood sales channel [not a Priceline/Expedia/Travel Agent rate]).

Of course you can ask for the weekend rate. I doubt that you'll get it though. Try fo find out if you want to stay during low season (e.g. check event/fair schedules for Roterdam; check for holidays in Dutch = not business travellers during that time; check the room rates for that period and compare with other periods; you can also try to find a low price if a main competitor has low prices. I would suggest this price only if anything else failed before. It sounds too much like pressure.). You can argue with that.