Joy and Christmas are often synonymous, but the connection is often assumed and not examined. At the same time, the repeated emphasis and flashing lights of "Joy" at Christmas can remain superficial and not produce true joy, but leave us feeling hollow and empty.
The Incarnation of Christ is the prerequisite for Joy. In Isaiah 35, we see a future age of everlasting joy and the reasons for it. Christmas is the dawn of this indestructible joy and the future is realized through Jesus.