What did the Romans call Halloween?

The joke you might be referring to is: “What did the Romans call Halloween? Gladia-‘tor’ Treat!”
This joke plays on the word “gladiator,” which refers to the fighters in ancient Roman arenas, and the phrase “trick or treat” which is commonly associated with Halloween. The humor arises from the unexpected combination of Roman culture with modern Halloween traditions. The word “gladia-‘tor'” is broken down to sound like “trick or” when said quickly, leading to the pun “Gladia-‘tor’ Treat” instead of “trick or treat.”