Well, I think Calvin overstates his case. The bible tells us very little about how or when to pray. Plainly, public prayer meetings were sanctionned (cf. I Cor 9-12), but other than Paul's guidance about what NOT to do, there's very little material provided for how they should be conducted. The crucial thing is that we not let the trappings of the holiday numb us to the wonders of the holyday.
I believe the only way this can be ensured is to recognize that there is no legitimate "holyday" called Christmas. The only holy day prescribed for God's people under the terms of the new covenant is the weekly Christian Sabbath. It is during the weekly observance that we come into the divine presence and find true wonder in His Being.
Other sorts of "piety" revolving around annual days of festivity and external forms have no place among the people of God. They are ripe for abuse.