Just as the Lord’s Day sanctifies the week, so also Eastertide sanctifies the Church yearEastertide—preceded by Lent—is the primary cycle of the liturgical calendar.  Within the Church year, this annual cycle is known as the Easter Cycle. And, as the primary cycle, the Easter Cycle interprets any other annual cycle within the Church year.
Secondary to the Easter Cycle is Christmastide, preceded by Advent—the Christmas Cycle.  This is true both theologically and historically.
Theologically, it is the resurrection which interprets the birth of Jesus.  Apart from the resurrection, Jesus has no more claim upon us than Socrates; Abraham Lincoln; Mohandas Gandhi; or Martin Luther King, Jr. Jesus would simply be one among many good leaders who managed to meet an unjust death.
Historically, two points buttress the Easter Cycle as that which give meaning and purpose to the Christmas Cycle.  First, that which is presumably the oldest of the four gospels (Mark) pays no attention whatsoever to the birth of Jesus, beginning instead with the account of his baptism.  And, Paul makes only passing references to Jesus’ birth.  Only later did Matthew and Luke attach enough importance to the nativity to comment upon it extensively.  Second, even more significantly, although the resurrection was obviously observed liturgically in the church from its very inception, the earliest recorded liturgical observance of Christmas day falls well into the fourth century.