When you celebrate an event before knowing it's final outcome. Often applied in sports, video games, and gambling, etc.
The idea behind the phrase is that children often tend to open their presents before 25th.
Kon: Guys! I think the Lakers are going to win in 4 this year!
Edgar: Definitely! Let's go drink some beer to celebrate!
after championship where Lakers won in 5.
Shyam: You guys fell for the Early Christmas Fallacy.
Next time just wait until they win before celebrating guys.