Exact opposite for me, a weekend day is explicitly a Costco day to rack up for the week or month. Anything else I have to do that day is incidental. I assume this is many people's experience too rather than the other way around.
But yeah the wrong day, and it's 30m at least.
(I have two good sized panniers and can end up with ~$150 of foodstuffs packed well in them no problem. More often than not, I get less than that and add more stops to the trip to pick up from 3-4 places while out. And I get my exercise while I'm at it.)
Prevents wasting money on things I don't need, and makes the trip quick.