Find Cheryl's birthday

Albert and Bernard became friends with Cheryl.
They wanted to know her birthday.
Cheryl gave 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 also does not know.
Bernard: At first I don't know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.

So, when is Cheryl's birthday?


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