Here are two explanations of the derivation of the word “Christmas.”
1. The word Christ stems from the Middle and Old English word Crist meaning the anointed one, the Lord’s Anointed.
It is borrowed from the Latin Christus and from the Greek Christos also meaning the anointed one. The Greek is a translation of Hebrew mashiach meaning anointed of the Lord or Messiah. In the word Christmas, the suffix -mas evolves from the Old English word maesse meaning festival, feast day or mass.
And from Wikipedia:
2. Christmas derivative of Christ and Mass.
Mass is the term used of the celebration of the Eucharist in the Latin rites of the Roman Catholic Church. The word itself is derived from the phrase with which the liturgical celebration concludes in Latin, Ite, missa est. In this phrase, the word missa is usually explained as late Latin for classical Latin missio, so that the phrase would be literally translated as: “Go, it is the sending.”
As much as I like Wikipedia’s explanation, I think the first one is the correct one. Having said that, what the second explanation loses on the swings, it gains on the roundabouts. What I mean is that even if it is an error, this error, although a loss, is always a-gain. Why? Let us assume that “mas” in Christmas does mean mass = sent. We then get “Christ sent”.
The Roman Catholic “Mass,” besides being the place where one receives the Eucharist, is also where one is given one’s marching orders – to go into the world and live and speak the Good News: Ite, missa est. Christ-mas then becomes a remembrance of the time when the Son of God was given His marching orders. Christ could have marched into our world at the head of a great army of angels, or erupted into the world like a missile, but instead he was born into this world as a helpless child. And died helpless a man(the Passion of Christ – passion in latin means “passive”).
The Word was made flesh and dwelt among us. And was torn for us. If you believe this, then the “us” is you.