Tom and Huck have set out for the supposedly haunted house, where they believe there's buried treasure to be found. But in their excitement at the prospect of finding all that lovely loot, they've forgotten something very important: the day of the week.
It's a Friday, which according to local superstition, is the unluckiest day of the week. It's certainly not the day you'd normally pick to go off looking for treasure. However, it's only when the boys rock up at the dead tree around noon to pick up their tools that they suddenly realize what day of the week it is.
Both boys believe in the superstition about Friday being the unluckiest day of the week. They're dead certain that they would've gotten into an almighty scrape had they searched for treasure on this particular day. Besides, Huck feels decidedly uneasy anyway, on account of a bad dream about rats he had the night before.
In that dream, the rats didn't fight, which means, according to Tom, that there's trouble around. But that shouldn't be a problem, he thinks, so long as he and Huck look "mighty sharp" and stay out of trouble. Among other things, this means no treasure hunting for today; they'll play games instead.