I’d have to do more digging, but I don’t think the Church ever formally inserted “obligation” to Sunday.
Its in the Code of Canon Law, RC.
To Wit:
Can. 1246 §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints.
§2. With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.
Can. 1247 On Sundays and other holy days of obligation, the faithful are obliged to participate in the Mass.
Moreover, they are to abstain from those works and aVairs which hinder the worship to be rendered to God, the joy proper to the Lord’s day, or the suitable relaxation of mind and body.