Harry woke up in a dreamlike state, contemplating the difference between where he used to be and where he was now. Back when he was a kid, Sundays meant being woken up early by his aunt and being supervised to make two full English breakfasts for his male relatives and a fresh fruit salad and poached egg for his aunt. It meant being forced to clean house while the Dursleys went to church to pretend to be the upstanding citizens they portrayed themselves as. He would first strip all the beds in the house, including the guest room where Marge occasionally stayed and start the linens. Then, he would do up the dishes and dust the numerous kitschy knick knacks Aunt Petunia collected and the wall of Dudley photos. When his relatives returned in the afternoon, he would play servant, fetching snacks and pouring drinks, meanwhile internally seething and counting down days until school started. For the rest of the day, he would pretend to be invisible. If he was silent and out of sight, the Dursleys were content to ignore his presence in their "normal" home.
Things were different with Sirius. For one, he was allowed to sleep in as late as he wanted, which still wasn't as late as his friend Ron might, but it felt luxurious. He'd come down stairs of the worn Pureblood house they were making into a home and help make a breakfast he was allowed to eat as much of as he wanted. Then, he'd spend time working on homework, asking questions about magic as the older man strummed his acoustic guitar. Maybe Remus and Tonks would stop by with baby Teddy and he'd get to be involved in a chain of godfather duties; Sirius parenting him and him helping to parent Teddy.
A noise in the hall shook Harry from his dream and reality came crashing back. He was in Grimmauld, yes, but he was alone and there were no happy memories. The war was five years over and neither Sirius, nor Remus or Tonks was there to see him grow into adulthood. He didn't even really have memories of Sirius helping him with homework. Those so called memories were just dreams of what could've been. With a despondent sigh, he rolled over in bed, staring at the ceiling. It was Sunday again and he didn't know how to spend the day. His curmudgeonly servant and housemate came muttering by his door and he knew he should get up. Maybe he would teach himself guitar on the one he had found in Sirius's room and kept because it hurt too much to throw any of his things away. Maybe he would make his own Sunday traditions now.