Neither Uncle Marius nor Harry had any objections to that, so James and Lily set to work instantly, borrowing an Astronomy book from Phineas Black's portrait in the library.
On a late night six days later, Harry sat in an upstairs room, nodding off in a plush armchair whilst he waited for Uncle Marius to return. Aunt Clytemnestra was at the country manor, organising the final details of their move to London. Mopsy the house elf had finished organising the house at number seventeen, Windermere Court, and Harry had spent most of the week conversing with his parents' portrait.
Two days before, Aunt Clytemnestra had brought over a portrait of Regina Malfoy and explained the situation to it. The portrait had been shocked at the circumstances under which Harry had grown up, and readily agreed to assist in any way it could., though she laughed at the idea that she would have a child with Sirius Black, a man she had got on quite well with, but never viewed romantically, much to the chagrin of Sirius's mother.
James and Lily's portrait had helped Regina to make an Unbreakable Vow not to tell anyone the truth about Harry's identity. (Uncle Marius explained that if a human tried to break an Unbreakable Vow he would die, but a portrait would simply be reduced to a blank canvas.) Regina told Harry all sorts of stories and other things he should know about her, and Harry practiced calling her 'Mum,' and his real parents 'Uncle James' and 'Aunt Lily,' since it had been agreed that they would be listed as his godparents.
As for Sirius Black, who was to be Harry's dad, it seemed that in the wizarding world portraits only began to talk once the people depicted had died, so Harry had no opportunity to speak with him. James's portrait, however, was more than happy to tell Harry stories about Sirius, and Uncle Marius managed to obtain a number of photographs, from which a handsome man with long, black hair waved happily. Harry made a point always to refer to Sirius as 'Dad.' It wouldn't do for him to forget. They were playing a very dangerous game.
When the door flew open and Uncle Marius finally walked in, a large flask in his hand, Harry thought he looked utterly exhausted.
'Do you want to wait and do this tomorrow?' Harry asked in concern.
Uncle Marius shook his head wearily. 'You have to drink the potion at the stroke of midnight,' he explained. He hung a solid silver cauldron over the fire in the fireplace and poured in the potion, which began to bubble and pop almost instantaneously. He added a bit of Harry's hair, as well as some of Regina's, which Aunt Clytemnestra had kept in an envelope, and some of Sirius's baby teeth, which Uncle Marius had narrowly managed to steal from the currently empty Black home at number twelve, Grimmauld Place.
He had only succeeded because Mopsy had distracted the Grimmauld Place house elf whilst the Squib slipped in under an invisibility cloak. Uncle Marius stirred the potion three times, then whipped out a piece of parchment and handed Harry a strange-looking quill.
'Write, "My name is Aries Sirius Black,"' he commanded. James and Lily had selected that name the day before.
Harry began to write and winced as he felt a pain in his hand. He continued regardless. Uncle Marius had warned him about the blood quill. When he had put the full stop, Uncle Marius took the parchment and dropped it into the potion.
The parchment dissolved instantly, and the potion changed to a light gold colour. Uncle Marius poured it into a silver goblet and they waited five minutes. At 11:59 he handed Harry the goblet, and as soon as the clock began to chime Harry drank it in one long draught, finishing the whole thing just as the clock struck twelve.
Nothing happened.
'Is something wrong, Uncle Marius?' Harry asked anxiously. 'I'm not changing.'
Uncle Marius laughed. 'It will take some time for the potion to spread through your system. Now run off to bed. I daresay you'll notice some changes in the morning.'
Harry was very tired, and so did not protest as Mopsy led him to his bedroom and tucked him in to his warm feather bed, skipping the now-usual bedtime story. Harry fell asleep in seconds and had a wonderful dream in which he played all night with a large, black dog.