The first written reference to a PB&J was in 1901. From the Boston Cooking School Magazine of Culinary Science and Domestic Economics: "For variety, some day try making little sandwiches, or bread fingers (!), or three very thin layers of bread and two of filling, one of peanut paste, whatever brand you prefer (JIF!), and currant or crab-apple jelly (YES!) for the other."
Know Your Meme dot com has a very thorough synopsis of the evolution of ”It’s Peanut Butter Jelly Time”, a beloved/ despised flash animation (created in 2002 by RalphWiggum and Comrade Flynn) that was paired with an accompanying song from the the Buckwheat Boyz. How sad that absolutely no one involved with this project wanted to use their real name.
An episode of “How I Met Your Mother” began with a scene in which Barney, Marshall and Ted are telling a dirty joke when Lily walks up. After the other guys refuse to tell her the joke, claiming it’s “boy-funny,” Barney whispers it to Lily, and it’s so dirty that she walks off and “didn’t speak to us for four weeks.” It was just the next stunt in a series of gags in which the show jokes about, rather than hides, the pregnancies of their two female leads. (Lily was presumably not in the rest of the episode due to maternity leave.) ANYway, the setup for the joke was “What’s the difference between peanut butter and jam?”