The implication seems to be that the Weasleys are perfectly ready for the game to last multiple days.
‘Look at the time,’ Mrs Weasley said suddenly, checking her wristwatch. ‘You really should be in bed, the whole lot of you, you’ll be up at the crack of dawn to get to the Cup. Harry, if you leave your school list out, I’ll get your things for you tomorrow in Diagon Alley. I’m getting everyone else’s. There might not be time after the World Cup, the match went on for five days last time.’
‘Wow – hope it does this time!’ said Harry enthusiastically.
‘Well, I certainly don’t,’ said Percy sanctimoniously. ‘I shudder to think what the state of my in-tray would be if I was away from work for five days.’
That being the case, the most obvious explanation is that they and their fellow wizards are planning to simply overstay, safe in the knowledge that with the muggle campsite owner's memory being altered so often (ten times a day), Mr Roberts is hardly going to notice a little thing like what day it is when he's already not noticed that that there are tens of thousands of wizards staying on his campground.