It is because the "Mass" comes from a poltical event when the Roman Empire determined in order to survive, they must control this new thing called christianity. They instantly converted the population in "Mass" gatherings. The gatherings were held on Sundays.
Uh, your story would be more believable had the Romans spoken English. They spoke Latin, however.
The term "Mass" as applied to a church service where the Eucharist is served derives from the Latin word for a dismissal (dimissio). After the Eucharist, the words missa est were spoken, signifying the formal end of the service.