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As with most things in life, I'd have to say "it depends." If you're renting at a place that's business-traveler heavy (think larger cities that aren't vacation destinations... IAH, PHL, LAX, Chicago, etc), you're probably best to get there later on Thursday after lots of business travelers have had the chance to return cars. If you're going somewhere that's vacation-friendly (LAS, MCO, etc), my logic would be that you're best to get there as early as possible on a Thursday before anyone else comes and claims what you might want.

Don't forget that a reservation isn't a guarantee that they'll have what you reserved! It's very possible that they could "upgrade" you into a 7-passenger SUV instead of a van, if they're sold out. Or they could just deny you when you show up. If it were me and I *needed* a minivan, I'd have reservations at 3 places and go to the cheapest when you arrive. If they're out, then work your way up the chain of companies 'til you find one with what you've reserved. It's kinda crummy to do this and throw off their fleet management, but it's also very crummy that they "know how to
take the reservation, [they] just don't know how to *hold* the reservation and that's really the most important part of the reservation, the holding. Anybody can just take them." (Seinfeld)