The day before Hermione had asked Sirius if he was free to run a few errands with her. She thought after their misunderstanding it might be nice to spend some quality time with each other trying to get things back to normal. She had detailed a four-step plan, which wasn’t incredibly sophisticated, but would do the trick of getting Sirius out of the house, so that the boys could bring in his birthday present and start setting it up. There was a tiny bit of tinkering that needed to be done so that the gift was functional within an all-magical home, and she wanted to give Fred and George some space to make it happen.