What was the comedic bit of "Romeo and Juliet?" Why did Shakespeare decide to use a comedic bit in the midst of such a somber point in the tale?
I assume you are talking about the death scene of Mercutio. This was a particularly somber and tense point in the play. After trying to hold off the fight between Mercutio and Tybalt, Romeo has just watched his friend get stabbed underneath his (Romeo's) own arm. As Mercutio is bleeding to death, he delivers his last speech. This speech curses and damns both Romeo and Tybalt for having been the catalyst that led Mercutio to his death. In the middle of this speech, he interjects some comedy:
“Ask for me tomorrow and you shall find me a grave man.”
This is a pun, or play on words. Mercutio makes it sounds like he will be emotionally serious (grave) tomorrow, but really he means he will be in a grave tomorrow.
Shakespeare does this first of all to stay true to the character - Mercutio has been flippant about things all along, why should death be any different? However, even more than this, the casual comedy sets up a contrast to how serious Mercutio has been, and how serious he still will be, in cursing the other men. He continues to say:
"A plague on both your houses!"
Without the moment provided by the pun, Mercutio's second repetition of the curse would run in together with the first. By breaking up the somber moment, Shakespeare gives more weight to it.