The angels descend from heaven and sing about the Almighty being the messengers of God and for our salvation. They were there before the foundations of the world and throughout the Word of God and history visiting people. The same angels that make our salvation their urgency descend down from heaven. Which things (those for our salvation) they look into and bring tidings of great joy (meaning preaching the Gospel) And many angels appeared and that joy beaks down to Ephrata's hills! Their song should reverberate in and through us!
To whom: Glory to God in the highest meaning in the highest the praise of God is there and breaks into the earth. The angels were there throughout time and understand about salvation as they saw all things unfold (including satan and the unfolding of salvation and Christmas even as they spoke about these things and announced them.) They announce the coming of Christ as they followed Him maticulously throughout history.
For what: Great tidings of great joy. It was the Gospel! Christ the Lord! This heart of the Gospel was here and would be the confession of the church throughout all the ages and is our confession till we sing it with them in heaven! He is born to you this day! That's why the message was "glory to God!" They sing the Gospel of PEACE! Peace on earth (understood very wrong often today.) This was the same Gospel that brings a sword to divide and to conquer! That comfort is that here warfare has been accomplished and we have received double for all our iniquities! It is the peace we have between us and God that all things are well between us and God as we sleep on our pillows each and every night!!! It is the peace of God that comes into our heart!
Concerning who: We must have this peace and song in our heart as we are the people of God!!! The SAME song! He has gathered all things INCLUDING us unto Himself in Christ and that IS CHRISTMAS!!!