If it's out of the blue, then there isn't really a reason to bring it up... If it's in response to "Let's celebrate...", how about "No, thanks. " or "Joe, I'm not really interested in celebrating 420..." I don't think you really need anything else.
To say politely, "It has no significance to me and probably never will. " I'm not sure we need a day to "celebrate" being a pot head. It's not the worst thing a person could be, but it's nothing to celebrate, either.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.