"SHHHH! Listen - come an' see me later, I'm not promisin' I'll tell yeh anythin', mind, but don' go rabbitin' about it in here, students aren' s'pposed ter know. They'll think I've told yeh --"
-- Hagrid, trying to shush the trio when they talk about the Stone (PS14)
When they let on that they know about Nicolas Flamel, the Philosopher’s Stone, and Fluffy, Hagrid tells them to come to his hut later and he’d tell them more about it.
The trio encounters Hagrid in the library a few weeks after this exchange:
"Hermione, the exams are ages away."
"Ten weeks," Hermione snapped. "That's not ages, that's like a second to Nicolas Flamel."
Counting back from the beginning of June, which seems to be when exams happen, we get the beginning of April. We know this event occurs a few weeks past that because the Easter break comes in between, which is two weeks long.