Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates. 

May 15      May 16    May 19
June 17     June 18
July 14      July 16
August 14  August 15  August 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.

Albert:      I don´t know when Cheryl´s birthday is, but I know that Bernard does not know too
Bernard:  At first I don´t know when Cheryl´s bitrhday is, but I know now.
Albert:     Then I also know when Cheryl´s birthday is.