Correct. If it is not sanctioned by the Word of God one cannot legitimately claim any "Christian piety" associated with its observance.
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.