Guy and Mary Burckhardt wake up in their house in Tylerton on June 15, having both had terrible nightmares, but they can't recall the events of their dreams. Guy dismisses the dream and goes to work as usual, the downtown offices of Contro Chemicals, which operates a highly automated and robot-staffed petrochemicals plant. But something isn't right; he is surrounded everywhere by loud and all-pervasive advertising jingles for everything from cigarettes to freezers.
A colleague named Swanson tries to speak to him but apparently doesn't get the desired reaction from Burckhardt and leaves. Burckhardt goes home, but the next morning, when he wakes up, he's had the same nightmare and the date is still June 15th. He knows what will happen when he gets to the office and that Swanson will again try to speak to him. This in fact happens.
That evening, Burckhardt discovers that his cellar has seemingly been dismantled and 'rebuilt', in a way he doesn't recognise. And the next morning, again it's June 15th, although he knows it can't be. He mentions this when Swanson again tries to speak to him. Swanson hustles him away to the empty halls of the chemical plant. Hiding in a room at the end of a long tunnel, he explains his theory that they have all been tampered with and that an invader has taken over the town for unknown reasons.
But neither of them are correct. It transpires that the chemical plant exploded. All the inhabitants of Tylerton were killed by the explosion or the chemical fumes. Dorchin, a ruthless advertising executive, took over the whole ruins and rebuilt them in miniature. The people were rebuilt as minuscule robots, and are being used as captive subjects for testing high-pressure advertising campaigns.

Question: Why does Burckhardt take time to listen to Swanson on the third day?
Answer: His cellar seems to have been dismantled and rebuit and the date is still the same.
Alternative answer: Because he realizes that it cannot be June 15 again.