[polak]

Originally Posted by darklord700

There's a second answer which Polak got depedning on the assumption you make.

Spoiler!

Alberta heard Bernard say Bernard does know, so Alberta knows it cannot be May 19 and June 18. It also cannot be June 17 otherwise Albert would have known at that stage.

Bernard at that stage knows the remaining possible dates are:

May 15, 16
July 14, 16
August, 14, 15 & 17

And since Bernard declares he know, it is only possible if Cheryl tells Bernard the 17th date, so August 17 is also the correct answer.

Haha this hurts my head. I tried to pick the "right one" many times and I confused myself with my own line of thinking multiple times and had to restart.