WHICH IS FOR CHRISTIANS; SATURDAY OR SUNDAY?

I have before me an article;

 "THE SABBATH MADE FOR MAN". In KINGDOM DIGEST (JULY 1989,PP 47-53). In this article much emphasis is given to the need today to return to obedience,to full obedience to God's Holy laws with special emphasis on THE FOURTH OR SABBATH COMMAND of Exodus 20:8-11 and deutronomy 5:12-15. Scripture text are given as to HOW TO KEEP THE SABBATH HOLY and the natural blessings promised if fully obeyed as commanded in the scripture. To this portion we say AMEN But according to this teachings, "the spirit of the law" does not require that we keep one certain day of the week;MEN OR NATIONS selects the seventh-day they wish to give to God---after the six days of work----Thus sunday becomes God's holy day when thus observed by most professed christians.this is reckoned as FULL obedience to the command of God to keep the sabbath day holy.nor do men break the fourth command by observing the first day of the week in a sequence of six days of labour followed by one day of rest. A redeemed nation will set aside a day as holy to the lord in conformity with the spirit of the law...
ÁŔĔ PŐĔPĹĔ ßŔĔÁĶĨŃĞ ŤĤĔ ŦŐÚŔŤĤ ČŐMMÁŃĎMĔŃŤ ŴĤĔŃ ŤĤĔŶ ŐߌĔŔVĔ ŚÚŃĎÁŶ (ŤĤĔ ŦĨŔŚŤ ĎÁŶ ŐŦ ŤĤĔ ŴĔĔĶ) ĨŃŚŤĔÁĎ ŐŦ ŤĤĔ ŚĔVĔŃŤĤ ĎÁŶ ÁŚ ČŐMMÁŃĎĔĎ?
what does the bible teach about making any change in God's laws? Can a man, a church or a nation set aside a day of their own choice and make it thus a holy day acceptable to God instead of the seventh day that God sanctified and made holy at creation?