Dialogflow doesn't have a lot of built-in conditional processing. So while you can use @sys.date to turn "today" into the date, you can't then use the date to figure out some other value.
You can, however, use the Fulfillment section to turn a date passed as a parameter into whatever you want using whatever logic is available. For a simple task like this, you can use the built-in Fulfillment Inline Editor. If you enable it, you'll be presented with some sample code, including an illustration how to access the parameters from the request.
Once you have deployed it, you'll also need to turn on Fulfillment for the specific Intent that is doing the processing. (See the second image below.)
(Update: It does have some conditional processing, since it can choose different Intents based on the Contexts that are set and specific utterances. However, there is no Context that is set for the current day of the week. While you can set one yourself, this would be done through Fulfillment, so it is probably just as simple as actually processing the parameter. It might be useful for a more extensive conversation, but not for a short question-answer.)

