Who Made Sunday Holy?

And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: Daniel 7:25

Question - Which is the Sabbath day?

Answer - Saturday is the Sabbath day.
Question - Why do we observe Sunday instead of Saturday?
Answer - We observe Sunday instead of Saturday because the Catholic Church, in the Council of Laodicea (A.D. 364), transferred the solemnity from Saturday to Sunday. 

Peter Geiermann, C.S.S.R., The Convert’s Catechism of Catholic Doctrine, p. 50, 3rd edition, 1957