Juneteenth is the same sort of portmanteau as Trucktober. Plus holidays have weird names. What's a Christmas, a Mardi Gras, a Festivus? It's almost entirely a matter of usage and familiarity.
And as an aside, I was curious about Festivus. Apparently it's Latin for "excellent, jovial, lively."
The writer of this episode based it on something from his family that his father did: https://www.youtube.com/watch?v=1oeu2cVABHg