After the night James and Lily Potter are killed and Voldemort disappears, Peter Pettigrew fakes his own death in a busy Muggle street. All the Ministry can find of Pettigrew is one finger. To throw them off his trail, Pettigrew had cut off his right index finger before transfiguring into his animagus rat form. Sirius Black is found at the scene, laughing (PA3). Sirius is arrested and at some point in the next few weeks is sent to Azkaban without a trial. He is 22 years old. (PA10)
It's about 90% certain this happened on November 1, but it's also possible it was November 2.
In PA3, Stan Shunpike tells Harry that Sirius "Laughed," ... "Jus' stood there an' laughed." Asked about this, JKR said: "The laughter - he was absolutely unhinged by James' death... He knew what he'd lost. It was a humourless laugh. Pettigrew, who they, in a slightly patronizing way, James and Sirius at least, who they allowed to hang around with them, it turned out that he was a better wizard than they knew. Turned out he was better at hiding secrets than they knew (TLC)."
Cutting off a finger here foreshadows Pettigrew sacrificing the rest of that hand for Voldemort's "rebirthing potion" (GF32) and Pettigrew dying for real when the magical replacement for that hand turned on him (like he had betrayed others) (DH23). -BB