Four months before the opening of The Empire Strikes Back, Luke Skywalker, C-3PO, R2-D2, and Chewbacca appeared as the special guest stars on The Muppet Show. Mark Hamill's first line as Skywalker is:
It seems we've landed on some sort of comedy variety show planet.
...and it goes downhill from there. The whole show is available on YouTube in three parts:
The appearance was probably orchestrated as a promotional crossover. Frank Oz voiced Yoda in Empire and was a lead puppeteer for The Muppet Show, performing Missy Piggy and Fozzie, among others.