Yes it is.
That is why it is a mortal sin to go shopping on Sundays and Holy Days, perform servile work on Sundays and Holy Days, unnecessarily travel on Sundays and Holy Days, and fail to go to Church to worship God on Sundays and Holy Days. All these things either detract from our own worship of God or prevent others from worshipping God by making them work.
I would agree with much of this except the primary point. Sunday is not the sabbath of the Lord.
What about this?
Colossians 2 v 13-16:
13When you were dead in your sins and in the uncircumcision of your sinful nature,[b] God made you[c] alive with Christ. He forgave us all our sins, 14having canceled the written code, with its regulations, that was against us and that stood opposed to us; he took it away, nailing it to the cross. 15And having disarmed the powers and authorities, he made a public spectacle of them, triumphing over them by the cross.[d]
16Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day