The 'trick' being- when he told Snape how to get into the Whomping Willow.I think I know the answer to this but I know other people have differing opinions.
So to kick off I think that it was in the 5th year but before the 'Snape's Worst memory' incident. The reason I think this is that Snape reminds Dumbledore that Sirius Black tried to kill him when he was 16. (so that's 5th or 6th year). Then in 'The Princes Tale' when Harry is seeing his memories- he sees the memory of Lily talking about the incident before he sees the 'Snape's Worst Memory'- thought again. That makes the 'trick' in 5th year.
I'm asking for opinions firstly, because I'm not sure pensieve memories would neccessarily be in chronological order (although they all seem to be in this chapter), but also because I've seen other people placing the incident in the 6th year.
Do we have definitive proof when it is? If I set it at the time I think it is will it get rejected for being non-canon?
Any opinions welcome.