God is no so much concern about days in which we worship Him for He even says in His word that man was not made because of Sabath but the Sabath for man. This means days are not more important to God than man. By the Holy Spirit each one of us should be decide on which day he wants to worship God as Paul said:
One man esteems one day above another: another esteems every day alike. Let every man be fully persuaded in his own mind. He that regards the day, regards it unto the Lord; and he that regards not the day, to the Lord he does not regard it. He that eats, eats to the Lord, for he gives God thanks; and he that eats not, to the Lord he eats not, and gives God thanks.Romans 14:5-6