A blog article titled "Dies Domini 47: A History of Sunday Obligation" points to Pope John Paul II Apostolic Letter Dies Domini (1998) paragraphs 46 to 49, providing:
a short history of Sunday Obligation expressed in church documents
when it became mortal sin
rationale and circumstances
Paragraphs 46-47 below:
Since the Eucharist is the very heart of ...
Remember that thou keep holy the sabbath day.
cf. Kellner's Heortology § 2. Sunday and its Observance as a Day of Rest (pp. 6-13)
Holy Office's March 4, 1679, condemned proposition 52
The precept of keeping feast days is not obligatory under pain of mortal sin, aside from scandal, if contempt be absent.