“Never see a pie before an auction”

SOOKIE: Because I’m baking for the picnic tomorrow and it’s supposed to be a surprise.
JACKSON: Oh sure, never see a pie before an auction, it’s bad luck.

Jackson is making a joke based on the old superstition that it is unlucky for the groom to see the bride’s wedding dress before their wedding. Not only is it a sign that he is thinking about marriage, it’s a possible foreshadowing of a future episode when a wedding dress is unluckily seen.