If you are casting a spell that requires more than an action to

Enter: Ritual spells These are free spells that don't consume a spell slot when they're cast Ritual spells can be obtained by any class

Characters with no spell slots at all can cast ritual spells thought the feat, so there is no reason that you'd need to have one available to do

does the pcie slot matter for gpu cast the spell gained as a ritual unless you already had ritual casting Casting the spell you gain from this feat does not use a spell slot

does slot open on sundays Basically, any spell can be cast as a ritual