Felis Påsguan Nochebuena
"Christmas" itself comes from the phrase "Christ's Mass." Back when England was Catholic, some feast days were named after the Mass of that day's feast. So, on the feast of Saint Michael, they celebrated Michaelmas. On the feast of the Purification, when candles were blessed, it was Candlemas. On the day of Christ's birth, it was Christmas.
Now the Marianas were influenced by Spain and its customs, and the Catholic religion. What we call Christmas in English is called the feast of the Nativity of the Lord in the Catholic Church. "Nativity" is a fancy word for "birth."
"Nativity" in Spanish is Natividad. Applied to Christmas, it is simply Navidad. Thus you have heard of Jose Feliciano's famous song Feliz Navidad (Merry Christmas). Feliz means "happy" or "merry."
But an older name for the feast of the Birth of Jesus is Pascua. Pascua is the term for one of three great events in the religious calendar : the Birth of Jesus, the Resurrection of Jesus and finally the coming of the Holy Spirit or Pentecost. All three feasts are considered pascua.
In order to distinguish these three, Christmas became known as Pascua de Nochebuena; Easter as Pascua Florida (or, "Flowery Pascua" on account of the Easter flowers that come out in spring) and Pentecost as Pascua de Pentecostés.
Nochebuena literally means "good night" and refers to December 24th, Christmas Eve, when the Birth of Jesus is first celebrated in church.
Believe it or not, despite all this linguistic technicality that may have you scratching your head, if indeed you are still reading, many man åmko' knew all of this! They were well-trained.
OK, now pascua became påsgua in Chamorro. So what is Feliz Pascua in Spanish becomes Felis Påsgua in Chamorro. You can add Nochebuena in there, too, to make it clear (remember, for the Spanish and the man åmko' Chamorro, there are three pascuas). So, Felis Påsguan Nochebuena.
As an alternative to all this, I lean toward Magof Nochebuena. This phrase keeps Nochebuena to denote Christmas, but uses magof instead of felis. Magof is pure Chamorro and means the same thing as felis, which is "happy." One could also say I suppose Magof Påsguan Nochebuena, but I think the shorter version accomplishes the same task.
Don't even ask me to translate that (to me) unsavory surrogate, else I will ask you to what holiday you are referring.