Why is the Catholic Church service called Mass?

It isn't, in the Catholic Church, "services" are called "Offices" like Morning Prayer (the older term was Lauds), and Evening Prayer (the older term was Vespers). Mass refers to the Eucharist which is NOT a service in the way that protestants and seculars think of as a Church service, that would be Lauds or Vespers, basically a prayer service with Bible readings and a homily. Mass refers to the re-presentation of Christ' sacrifice of Himself on the cross, making the graces of that sacrifice available to the people present. The nearest thing that the protestants would have to it would be a Communion service, but, again, it is not a service, it is the most holy sacrifice of the Mass: the Eucharist, when Our Blessed Lord sacrificed Himself on the cross for our redemption, and gives us His Body and Blood to eat and drink. The term "Mass" is derived from the Late Latin Word missa(dismissal), a Word used in the concluding formula of Mass in Latin: "Ite, missa est" ("Go; it is the dismissal").