An essential doctrine of Reformed Christian worship is a derivative of the Protestant tenet sola Scriptura called the Regulative Principle. Simply put, the Regulative Principle teaches that God alone has the right to institute the acceptable way of worship. We find this doctrine in the Westminster Confession of Faith 21.1 which reads, "The acceptable way of worshipping the true God is instituted by himself, and so limited by his own revealed will, that he may not be worshipped according to the imaginations and devices of men, or the suggestions of Satan, under any visible representation or any other way not prescribed in the holy Scripture."
The basic thought is this: Worship is fundamentally prostration before God. In worship we seek to be submitted to God's holy word, receiving the Father, the Son, and the Holy Spirit as our blessedness and reward in accordance with his covenant of grace. But being submitted to God in worship not only includes submission with regard to what and who we worship but also with regard to how we worship. In other words, worshipping God in a way he has not prescribed is tantamount to worshipping a false God. Or put another way, worship that is not regulated by God cannot be worship that is glorifying to God. It is what the framers of the confession called "will worship," since the human will is exalted to a position of authority over God.
Just as God himself is set apart as the only acceptable object of worship, so also God has set apart peculiar means, a peculiar manner, and a particular time for his worship. We see these aspects of worship in the first table of the moral law (i.e. The Ten Commandments, Ex. 20, Deut. 5):
1. OBJECT set apart for worship. You shall have no other God's before me.
2. MEANS set apart for worship. You shall not make any graven images.
3. MANNER set apart for worship. You shall not take the name of the Lord your God in vain.
4. TIME set apart for worship. Remember the sabbath day to keep it holy.
These basic principles are rooted in the character of God. They are unchanging. They are revealed to us for his glory and our good.
With repect to the when of worship, in Old Testament times God prescribed the weekly sabbath day (i.e. the fourth commandment) along with other holy days. The fourth commandment is essentially moral, prescribing the one-day-in-seven Sabbath. But it also has a ceremonial aspect, namely the prescription of Saturday. In New Testament times the ceremonial law has been abrogated, and the ceremonial aspect of the fourth commandment has changed from Saturday to Sunday. But the moral essence of the fourth commandment (i.e. one day in seven) continues forever. The Christian Sabbath, Sunday, the Lord's Day, is the only day set apart as holy in the New testament era. God has graciously given his church this day alone for regular public worship. Public worship on any other day, which is certainly permissible, should be extraordinary, irregular. In other words, the Christian calendar is a weekly calendar--six days you shall work and one day you shall rest. It is not yearly (or beyond) as in Old Testament times.
So, with respect to Christmas (Christ-mass), the problem is not that Christ is being taken out of Christmas as is often suggested. The problem is that he was never there to begin with. It's not taking Christ out of Christmas that troubles me; it's putting him in it.
I think it's great for families, schools, workplaces, and nations to set apart times in which people can come together, exchange gifts, and feast. I do things like that with my family and others every year for Thanksgiving, Christmas, birthdays, anniversaries, etc. I also think its fine for churches to meet irregularly for public worship on days other than Sunday. But we should be very careful not to presume to be more wise than God when it comes to observing holy days. We shouldn't presume that any day or season has religious significance if God has not given it such. God alone has authority to set apart days as holy, and he has not seen fit to do that with any day but the weekly sabbath since the coming of Christ.