Santa's Big Sack

Listen up, buttercup. The joke is about why Santa's sack is so big. The punchline is that he "comes once a year", which is a sneaky way of saying Santa only visits once a year, but also kinda implies a sexual thing. It's funny because it's unexpected and a little bit naughty. You probably didn't get it because you are one of those guys who clap when the plane lands.