Can you back up your claim that Sunday is a Roman Catholic Institution?

Answer: Yes, we can. Here are some of the claims of the Roman Catholic Church in this regard:

“Sunday is a Catholic institution, and its claims to observance can be defended only on Catholic principle… From beginning to end of Scripture there is not a single passage that warrants the transfer of weekly public worship from the last day of the week to the first.” –The Catholic Press, Sydney, Australia, August, 1900.

• “Question – Have you any other way of proving that the (Catholic) Church has power to institute festivals of precept (to command holy days)?”

“Answer – Had she not such power, she could not have done that in which all modern religionists agree with her: she could not have substituted the observance of Sunday the first day of the week, for the observance of Saturday the seventh day of the week, a change for which there is no Scriptural authority.” – Stephan Keenan, A Doctrinal Catechism, page 176.

• “Perhaps the boldest thing, the most revolutionary change the Church ever did, happened [when] the holy day, the Sabbath, was changed from Saturday to Sunday. ‘The Day of the Lord’ (dies Dominica) was chosen, not from any directions noted in the Scriptures, but from the Church’s sense of its own power” -Sentinel, Saint Catherine Catholic Church, Algonac, Michigan, Volume 50, Number 22, May 21, 1995
  
  

Additional Reading:

• A Remarkable Symbol
• Institution of the Sabbath
• Yahuwah's Memorial