In reply to:
what is exactly "mas"?
A suffix, shortened from "mass", as in Christmas = Christ mass, or the feast day of Christ. There is also the somewhat archaic Martinmas = Martin mass, or St. Martin's feast day (Nov. 11, I believe -- it's not a red letter day in the U.S.) So Candlemas = candle mass, the feast day on which candles are blessed for the coming year, actually the Feast of the Purification of the Blessed Virgin Mary, or The Feast of the Presentation of Our Lord in the Temple (it has been called by both names).
And before you ask, a red-letter day is a major church holiday. It comes from the way the church Kalendar was printed in missals and prayerbooks. The major feasts were printed, like the rubrics, in red, the others in black; hence, a major feast was often referred to as a red-letter day. A rubric, by the way, is a directional note concerning what is to be done in, before, or after a ceremony or rite, and is printed in red rather than black to indicate it is not part of the text of the rite.
