This seems so obvious now, but it never really clicked before.
“It’s very lucky he picked tonight, you know,” said Hermione as they climbed fully dressed into their sleeping bags and propped themselves on their elbows to talk. “The one night we weren’t in the tower…”
“I reckon he’s lost track of time, being on the run,” said Ron. “Didn’t realize it was Halloween. Otherwise he’d have come bursting in here.”
OF COURSE HE REALIZED IT WAS HALLOWEEN
OF COURSE SIRIUS WENT AFTER PETER ON THE ANNIVERSARY OF JAMES AND LILY’S DEATH
I always thought he wanted an empty tower, so he picked a day he knew the students had something going on, but NO IT WAS HALLOWEEN AND SIRIUS IS ANGRY AND STILL GRIEVING AND HE NEEDS REVENGE ON PETER, AND HE NEEDS IT THEN.