Observing Sunday is as ancient as anything Christian. Vatican II emphasized it:
3. The fundamental importance of Sunday has been recognized through two thousand years of history and was emphatically restated by the Second Vatican Council: “Every seven days, the Church celebrates the Easter mystery. This is a tradition going back to the Apostles, taking its origin from the actual day of Christ’s Resurrection — a day thus appropriately designated ‘the Lord’s Day’.”(Sacrosanctum Concilium 106)
The pope who oversaw most of the council affirmed this in the promulgation of liturgical reform:
Paul VI emphasized this importance once more when he approved the new General Roman Calendar and the Universal Norms which regulate the ordering of the Liturgical Year.(Cf. Motu Proprio Mysterii Paschalis (14 February 1969): AAS 61 (1969), 222-226)
The coming of the third Christian millennium brings another occasion for another pope, John Paul II, to bring this concern to the Church:
The coming of the Third Millennium, which calls believers to reflect upon the course of history in the light of Christ, also invites them to rediscover with new intensity the meaning of Sunday: its “mystery”, its celebration, its significance for Christian and human life.
Pope John Paul II saw this letter as an affirmation of what other bishops had to say in the years previous, 1963-1998. Bishops are intended to be part of the audience here. The Holy Father then clearly based his writings on his own experience as a bishop, and his intent here was to encourage other bishops to place some priority on this matter as a teachable moment as well as a spiritual opportunity.
I note with pleasure that in the years since the Council this important theme has prompted not only many interventions by you, dear Brother Bishops, as teachers of the faith, but also different pastoral strategies which — with the support of your clergy — you have developed either individually or jointly. On the threshold of the Great Jubilee of the Year 2000, it has been my wish to offer you this Apostolic Letter in order to support your pastoral efforts in this vital area.
This letter is also for us laity:
But at the same time I wish to turn to all of you, Christ’s faithful, as though I were spiritually present in all the communities in which you gather with your Pastors each Sunday to celebrate the Eucharist and “the Lord’s Day”. Many of the insights and intuitions which prompt this Apostolic Letter have grown from my episcopal service in Krakow and, since the time when I assumed the ministry of Bishop of Rome and Successor of Peter, in the visits to the Roman parishes which I have made regularly on the Sundays of the different seasons of the Liturgical Year. I see this Letter as continuing the lively exchange which I am always happy to have with the faithful, as I reflect with you on the meaning of Sunday and underline the reasons for living Sunday as truly “the Lord’s Day”, also in the changing circumstances of our own times.