In a sermon dated November 6 1863, Brigham Young explicitly taught that he did not believe that Sunday could be the only day of the week LDS could hold their weekly services on (cf. the practice of having Church services on Fridays in Muslim countries and Saturday in Israel):
You may ask me if I always keep the Sabbath holy. When I am out travelling and happen to be where there is not a convenient place to observe the Sabbath, I sometimes travel on that day, and content myself with making a Sabbath of the second, third, fourth, fifth or sixth day of the week. But generally, when I am travelling, it is all the time Sabbath to me, for it is a constant succession of meetings. But do I send my teams to the kanyons on the Sabbath day? No, that is if I have the control over that which should be under my control. I keep the Sabbath, and I wish to keep all the laws of God, and there are a great many of this people that do so; but many of them go astray in failing to observe these things—in not paying their Tithing faithfully, in not keeping the Sabbath day holy, in not saying their prayers constantly, in not feeding the poor and clothing the naked, and in not administering to the Priesthood. As a people we do as well as we can do, but some do not do as well as they might do. (JOD 10:284)
For more, see the powerpoint slides for a presentation I gave on the issue: