Pharaoh's birthday. Birth-days by persons of eminency then were, and since have usually been, celebrated with feasting and rejoicing.
Related Commentaries of Genesis 40:20
Genesis 40:20
20 And it came to pass the third day, which was Pharaoh's birthday, that he made a feast unto all his servants: and he lifted up the head of the chief butler and of the chief baker among his servants.