Dumbledore wants Horace Slughorn to return to the Hogwarts teaching staff. The headmaster brings the famous Boy Who Lived along to entice Slughorn to accept. The strategy works.
En route to the Burrow from Privet Drive, Dumbledore takes Harry to call on Horace Slughorn – who, in trying to evade the Death Eaters, possible employers, and anyone who would make him do work or associate himself with one side over another in a conflict, has been moving to a different Muggle home every few days. Knowing that Dumbledore is coming to ask him to return to teaching at Hogwarts, Slughorn quickly stages his temporary residence so that it appears as though a violent intrusion by Death Eaters has occurred. However, Slughorn forgets to put a Dark Mark in the sky above the house, and it is apparent to Dumbledore that it is a ruse. Inside, Dumbledore easily locates the armchair that is really Slughorn in disguise, and turns him back to his human shape. After the two old colleagues clean up the mess with magic and Slughorn steadfastly refuses to go back to teaching, Dumbledore leaves Slughorn and Harry alone. Slughorn boasts to Harry about successful former students of his with whom he keeps in touch and gets perks. He also displays a questionable attitude regarding Muggle-borns; apparently liking and accepting individual ones (such as Harry’s mother), but, on the whole, doubting their competency as witches and wizards. Just as Harry and Dumbledore are about to leave when Slughorn decides that collecting Harry and being safe at Hogwarts outweigh the downsides of Dumbledore’s request, and he takes the job.
Dumbledore called on the Dursleys at midnight (an hour after the time he wrote that he would be there, which was 11 p.m.) After that conversation, he and Harry left for Budleigh Babberton, which means that they arrived there around 1 in the morning on the 13th of July.
As Dumbledore and Harry walk to the house in which Slughorn is staying, Dumbledore remarks that Voldemort "killed enough people to make an army of." In the sea cave months later, they discover that Voldemort does, indeed, have quite a sizable group of corpses ready to do his bidding (HBP26). -BB