Keeping Sunday as a day of rest is not only a liturgical principle. Having its roots as a Jewish Sabbath, an observance and celebration of creation as well as a commemoration of liberation from slavery, there were secular aspects in Israelite and Jewish tradition, to be sure. Pope John Paul II reminds us that the observance of Sunday is very much one of justice for people who work, and who work hard:
66. Finally, it should not be forgotten that even in our own day work is very oppressive for many people, either because of miserable working conditions and long hours — especially in the poorer regions of the world — or because of the persistence in economically more developed societies of too many cases of injustice and exploitation of (people) by (people). When, through the centuries, she has made laws concerning Sunday rest,* the Church has had in mind above all the work of servants and workers, certainly not because this work was any less worthy when compared to the spiritual requirements of Sunday observance, but rather because it needed greater regulation to lighten its burden and thus enable everyone to keep the Lord’s Day holy. In this matter, my predecessor Pope Leo XIII in his Encyclical Rerum Novarum spoke of Sunday rest as a worker’s right which the State must guarantee. (Cf. Encyclical Letter Rerum Novarum (15 May 1891): Acta Leonis XIII 11 (1891), 127-128)
*The most ancient text of this kind is can. 29 of the Council of Laodicea (second half of the fourth century): Mansi, II, 569-570. From the sixth to the ninth century, many Councils prohibited “opera ruralia“. The legislation on prohibited activities, supported by civil laws, became increasingly detailed.
So it can be a matter of religious freedom, too. But not only that. There is something essentially human in taking time to rest. And more, in being able to exist and function as a fully-endowed adult person by partaking in the rhythm of work and rest, of providing for self and loved ones, but also partaking of leisure which is very much part of what makes us human:
In our own historical context there remains the obligation to ensure that everyone can enjoy the freedom, rest and relaxation which human dignity requires, together with the associated religious, family, cultural and interpersonal needs which are difficult to meet if there is no guarantee of at least one day of the week on which people can both rest and celebrate. Naturally, this right of workers to rest presupposes their right to work and, as we reflect on the question of the Christian understanding of Sunday, we cannot but recall with a deep sense of solidarity the hardship of countless men and women who, because of the lack of jobs, are forced to remain inactive on workdays as well.