The Potters invited the three Marauders for Harry's first birthday on 31st July, 1981.
There was an Order meeting that afternoon. Dumbledore dropped in towards the end.
"I have got a letter from James. He says Godric Hollow has been surrounded by Death Eaters in disguise. They haven't been able to break through the enchantments yet, but I think our spy has struck again."
In the silence that followed, Remus caught Sirius looking at him strangely, though he did not understand why.
"Remus, Peter, Sirius-you are going there for Harry's birthday today, aren't you? Please inform James and Lily that I'll drop in late. I've got something important to discuss with them."
"Right, sir," said Remus.
The three of them went to Godric Hollow together.
"Look," whispered Peter. They all saw an unnaturally large number of people standing around the area.
"Voldemort must want Harry really bad," said Remus in a worried tone.
"Yeah," said Sirius, his voice dripping with venom. As Remus looked up, he was stunned to see Sirius glaring at him.
Suddenly it struck Remus what Sirius' behavior in recent days meant. The realisation that Sirius thought he was the spy hurt him worse than anything had ever done. But he couldn't bring himself to say anything.
As they entered, Harry, dressed up smartly for the occasion, came running towards them, crying in his adorable voice, "Unco'o Moo'ey! Pa'foo'!"
"Happy birthday! How're you, little Prongs?" said Sirius, grinning and swinging him up in his arms.
"Fi'e. How you?"
For some time, Sirius and Remus swooned on Harry. James and Lily came out, and the house rang with laughter and happiness, which continued till Dumbledore came.
--------------------
"The Fidelius Charm?" said Lily. "It's something that's used only in the worst of situations."
"I don't want to scare you, Lily, but your situation is really bad. It is only a matter of time before they break through the enchantments."
"So we choose a Secret Keeper," said James. "And unless he gives us away, no one in the world can find us."
"That's right," said Dumbledore.
"I think it's a great idea. We can keep James, Lily and Harry absolutely safe," said Sirius. "When do we perform the charm?"
"Not any time very soon. I need you to completely understand the implications of the charm," said Dumbledore, producing a book. "This book contains everything about the charm. You will need a month at least. Also, I want you to decide very carefully who your Secret Keeper will be."
James and Lily exchanged a glance.
"For now, I will strengthen the protection on this house. I will also check on you from time to time, till you are ready to perform the charm. "
Later that night, when everyone had left, James asked his wife, "We know who our Secret Keeper will be, right?"
"Of course, Sirius is our natural choice. But Remus too..." said Lily.
"We can't choose both of them, Lils, even if we trust them equally."
"You're right. We'll have Sirius."