Actually the word "merry" in the context of "Merry Christmas" means peaceful or pleasant. It's from old English. We are familiar with the carol God Rest Ye Merry Gentlemen. So, we are wishing others to have a nice, peaceful Christmas, not a boisterous, fun-filled one.
It is true that Christmas is perhaps the most joyous time of year for Christians and even for some nonbelievers. Almost everyone remembers the happiness of their childhood Christmases. I think that this is why many of the secular traditions are maintained. So, many decorate (some extravagantly), send Christmas cards, listen to and sing carols, get together with family and friends, go to parties and eat well. A few have tried to maintain this spirit of joy while denying the Person responsible for it, Jesus Christ.
Christmas means Christ's Mass. It is the day on which the Mass of Christ is offered. Yet, every Mass is that of Jesus Christ. Every Mass is the unbloody Sacrifice of Jesus on Calvary whereby He redeemed us from our sins. The name Christmas was given to underscore the Mass commemorating his Birth. It is because of this Birth that the Redemption could take place.
So, Jesus is the reason for all the joy, giving and receiving. Without Him the celebration is empty, meaningless. The faithful Catholic keeps Jesus central in their life. Thus the happiness of Christmas day is real and lasting -- lasting into eternity.
Knowing this, let each of us make every effort to cooperate with God's grace and live our Faith. We should be in conversation with God and always consider how He wants us to act. It is good to remember that He sees us 24/7, not only to judge us but to help us -- to help us be like His Son, for whose birth we are most grateful!