when did the prank take place?

[pippin_999]

No: HPFGUIDX 83284

Kneasy:
> It also causes me to wonder where the Gang got the idea of 
becoming  Animagi<

Remus says they got it from him: " They did something for me 
that would not only make my transformations not only bearable, 
but the best times of my life. They became Animagi <snip> 
[Dumbledore] never knew I had led three fellow students into 
becoming Animagi illegally."PoA 18

It makes sense to me that Remus, who would naturally try to 
learn all he could about his condition, would have discovered the 
information  about  werewolves and the Animagus 
transformation. 

My belief is that the prank happened in 6th year after the P2 
incident. We don't know that Snape entered the tunnel of free will. 
He could have been Imperius'd or confunded. But James did risk 
his life by entering the tunnel even if he could already do the 
transformation. There wouldn't have been room for an antlered 
stag to maneuver in the tunnel and Lupin would have torn him 
apart to get at Snape.

Pippin