If a person question whether the Sabbath is Saturday or Sunday the dictionary can definitely be a valuable resource alone with many other resources that tell about the men who changed the calendar from God to man.
For your knowledge and understanding:
Sunday is defined as the first day of the week
Saturday is defined as the last day of the week
A week is defined as a period of seven days from Sunday to Saturday
Man made Sunday their day of worship the unchangable God commanded Saturday His Blessed(Holy)day of worship choose how you worship whether of man or of God
Exodus20:8 Remember the Sabbath Day to keep it Holy unto God
Exodus35:2 Six days shall work be done on the seventh day there shall be to you a Holy day a Sabbath of worship unto God