Why is it called Christmas day?

Answer 1

Several church feastdays end in the suffix ~mas. It is shortened from the original Masse... meaning "the Mass of", so that Christmas means the Mass of Christ, and Martinmas means the Mass of St Martin, and Michealmas means the Mass of St Micheal.

The Christian Mass is varied slightly to incorporate a reference to the saint whose feastday is being celebrated. Some feastdays were so notable that they gained public recognition in the common terminology used in everyday life.... generally because fairs and markets were held on those particular days in medieval times.