I am not agreeing the way Esaias puts it and how it relates things with the Sabbath keeping, but I agree he is right in some parts.
The Israelites could celebrate the deliverance from Egypt every day of their life, and probably should, in remembrance. However, there was one single day that they had to do something special about it. That's called a
feast. And because the feast was related to the covenant between the Lord and them, it is called a
covenant feast.
Covenant feasts are act of worship to the Lord, and they have to be acceptable to him. The Lord picked the day, not them. The Lord commanded them to do it, they didn't innovate picking a day. Do you remember the sin of Jeroboam? Or do you remember the strange fire sin of the son of Aaron? You don't innovate with these things. You don't add or remove from it.
The Bible also talks about feast not directly related to the covenant of God, for example, the feast of Purim. Those are feasts of national (or community) significance.
Thanksgiving feasts. They are not covenant feasts. Those are your initiative. Nothing wrong with celebrating them. You can include here things like thanksgiving day, anniversaries, etc...
What is Christmas? As the Israelites with the deliverance from Egypt, you could mediate on the coming of the Lord in flesh every day of your life, but in one day you are making it something special. That's called a feast. Since the topic is in relation with the covenant of the Lord with us, the feast
pretends to be a covenant feast. This is the problem:
it is an innovation, it is the sin of Jeroboam, a strange fire.
It is not an acceptable worship to the Lord.
Not only that, but people decorate their houses in the name of the Christmas unto the Lord. The decoration is also an act of worship. Funny thing is that it is exactly what pagans do with their gods:
Jeremiah 10:3-4 (NKJV) 3 For the customs of the peoples are futile; For one cuts a tree from the forest, The work of the hands of the workman, with the ax. 4 They decorate it with silver and gold; They fasten it with nails and hammers So that it will not topple.
As Deut says, don't do to your God what they do with their gods. Picking the day to feast is not only wrong worship, but also the decorating is wrong worship.
You worship God in Spirit and Truth, and by presenting your body as a living sacrifice, holy, acceptable to God, which is your reasonable service. You ought to decorate your soul with good works, not your house unto the Lord. We make our house nice and we decorate it to make ourselves comfortable, and make guests welcome and comfortable as well. We do it for ourselves as humans that we are, with moderation, but we shouldn't do it as an act of worship to God, to celebrate an invented covenant feast.