Dumbledore recalled the order of the phoenix (an organisation he had made in the first wizarding war to fight Voldemort) and made 12 Grimmauld place it’s headquarters. Sirius had to stay in the house he grew up in, which he hated, and was not allowed to go outside at all because if he was caught by the ministry, he would be put back into Azkaban or possibly have to suffer the dementors' kiss. Ron and Hermione came to stay there early in the holidays, but Harry had to come much later and with a load of people protecting him. Harry stayed there for the rest of the holidays. Kreacher was still at Sirius’s house, and was always muttering about how he hated that the house was being used for headquarters to fight against Voldemort, because all of the Blacks (except Sirius) had thought that Voldemort was doing the right thing and they were on his side.

Harry went to school and Sirius was lonely again, apart from when the members of the order came in and out. Harry, Ron and Hermione came back for the holidays, and also when something bad happened to Mr Weasley. Mr Weasley got attacked by Nagini -Voldemort's snake- and would have died if Harry hadn’t seen it happen in his dream and tell Dumbledore, so he could get help. Hermione didn’t come back to the order when this happened, because she was not directly involved in it, but Harry, Ron, Ginny, Fred and George all did. Mr Weasley was alright, but he had to spend a long time in St Mungo's hospital for magical maladies and injuries. 

One day, Harry got told he had to do occlumency lessons with Snape, which he was not happy about.