Galatians 4:8-11 (ESV)
Formerly, when you did not know God, you were enslaved to those that by nature are not gods.  But now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more?  You observe days and months and seasons and years!  I am afraid I may have labored over you in vain.
I have more than once heard this leveled at us Lutheran’s by Biblically illiterate Baptists, as a charge against us for having a Liturgical year. So I want to make this very clear, we are free to worship God on what ever day we want. What Paul is getting at here is that we don’t have to celebrate the festivals of the Old Testament. This is fairly clear given the overall context of Galatians in which Paul rails against those who want to enslave the Galatians to circumcision and all that entails. This passage if anything needs to be leveled at Seventh Day Adventists who insist that Saturday remain our Sabbath. They need to hear what Paul writes to the Colossians: “Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath.” (Col. 2:16 (ESV)
We are free. No one has the right to tell us when we can or can’t worship God. The fact is that the Early Christians even in the New Testament had started worshiping on Sunday, because that was the day that Jesus rose from the dead. It made sense, and makes sense to commemorate that greatest of days every week with the Divine Service. We do this to celebrate the freedom from the law that Christ won for us by rising from the dead on Sunday. The “Super Apostles” had a problem with that freedom.
But we are also free as Christians to set times to commemorate certain days. It really doesn’t matter if Christ was born on the 25th of December, we celebrate the incarnation of our Lord (much more than a birthday party for Jesus!) on that day. It doesn’t matter if Good Friday doesn’t match up with the Passover. And now that it is so engrained in our society that even atheists celebrate Christmas on the 25th of December, we would be foolish to change the date. But if you can’t make it to church on Sunday there is nothing wrong with dropping in on a Wednesday night service instead.