Why don't people celebrate Christ's birthday in September--when he was actually born?

IBecause it's not a Birthday Party. It's a sacred memorial, appropriately set in context within the Liturgical year ...

The custom of celebrating the anniversary date of someone's birth is quite modern. The custom of celebrating the story of Salvation through an annual pageant of memorials is ancient; it is in fact Scriptural. The ancient Israelites were commanded to keep the Festival of Passover (Exodus 12:1-27), the Festival of Weeks (Exodus 23:16), the Festival of Tabernacles (Exodus 23:16). They were to keep these feasts every year, and use them as a teaching opportunity, to tell their children the story of their salvation from slavery in Egypt, and to remind the people that the Promised Land itself, down to the very fruits of the harvest, were a sacred gift from God.

The ancient Christian church recognized that they were Gentiles, not Jews. Scripture told the Gentile Christians the story of God's salvation, including most significantly God's promise of a messiah; but accepting Christ and reading Scripture did not make the Jewish story of the Exodus and the Promised Land their story. They took the Scriptural pattern of telling the story of salvation through annual memorials, but the story they told was the story of the New Covenant, not the Old.

Within the ancient liturgical year, Advent commemorates the Old Covenant. Christmas commemorates the Incarnation. Epiphany commemorates Christ's ministry. Lent commemorates Christ's passion. Easter commemorates the Resurrection. Pentecost commemorates the mission of the Church. And then we start again, about four weeks from now, with Advent again. It is how we live our daily lives immersed in the story of salvation; how we teach our children the story in a way that sinks deeply into their being.

Christmas comes when it comes because that's where it fits in. We tell the story of the birth of Christ so that we can go on and tell the story of Christ's life, teaching, death and resurrection. Story-telling is natural to both eastern and Celtic culture in a way that perhaps seems strange to modern westerners. It is a powerful tool in living the Gospel. If it's not a tool that is natural to you, or not a tool that fits in to your culture, don't use it. But give the benefit of the doubt to those of us that do use it. We are using it to fulfil the Great Commandment.