I won't give you the literal translation because a) I have a policy against doing people's homework for them, and b) you can figure it out with any Spanish-English dictionary.
Many proverbs (Spanish refranes) do require some knowledge of the culture to understand, but this probably isn't one of them. What you need to do is think of the language and word meanings in the broadest possible sense, a figurative sense, rather than rely on the literal meaning. If you can do that, you may see that this proverb really doesn't have much to do with comedy, just as "the early bird gets the worm" doesn't really have anything to with birds.
In this case, víspera, as any dictionary will tell you, has the literal meaning of "eve" or "the night before." But both víspera and its closest English synonym can be used in a broader sense. You're probably too young to remember the old protest song that said "we're on the eve of destruction," but that's a case where "eve" didn't have anything to do with the literal night before an event; rather it was used to mean that we're in the final stages of whatever comes before destruction. So the figurative meaning can be "prelude" or "that which comes before something else"; a loose translation of the refrán would be something to the effect of "humor comes before the truth."
As to the English equivalent, I can't think of one off-hand. The closest I can come up with is "In every joke there's a grain of truth," or better yet, "in every joke there's a germ of truth," but there very well could be one that's closer. Use your imagination. When translating from one language to another, it's important to translate ideas and concepts, not words. That principle is doubly true when it comes to understanding proverbs or refranes.
Edited to add: After this article was published, a reader suggested this English equivalent: "Many a true word is spoken in jest." I think that captures the spirit of the Spanish proverb quite well, even if the interpretation is far from literal.
