This is the 17th most frequent Greek word.
μας
"Μας" means "our" or "us" in English.
In this sentence, "μας" is used as a possessive pronoun indicating 'our' describing the celebration.
Αυτή είναι η γιορτή μας.
This is our celebration.
Here, "μας" functions as an object pronoun meaning 'us', referring to the group being addressed by the teacher.
Ο δάσκαλος μας είπε κάτι σημαντικό.
The teacher told us something important.
In this example, "μας" is also an object pronoun meaning 'us', referring to the recipients of the book.