Recall De Morgan's Law:

In this case,

Therefore,

Hence, the required negation follows from De Morgan's Law as:

The required negation is:
(A) "We are not having pie or it is not Jessica's birthday"
(B) The given statement is:
It is not the case that "walnuts grow on lily pads or caterpillars turn into butterflies"
By using De Morgan's Law, the equivalent statement is as follows:
Walnuts don't grow on lily pads and caterpillars don't turn into butterflies