One of the Ten Commandents (which Mormons follow) is to keep the sabbath day holy. In Exodus 20, it reads:
8 Remember the sabbath day, to keep it holy.
9 Six days shalt thou labour, and do all thy work:
10 But the seventh day is the sabbath of the Lord thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
11 For in six days the Lord made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the Lord blessed the sabbath day, and hallowed it.
For Mormons, this means that Sunday is to be a day of rest and not a day for all of our normal activities.
Practically, this means that Mormons don’t work, shop, go out to eat, go to the movies, etc. on Sundays. Instead, Mormon families attend church meetings, rest, read, and generally spend this time doing family activities. In today’s world that is increasingly busy, time with family is even harder to find so Sundays can be a wonderful break from the normal routine.
Elder John H. Groberg gave insight on the blessings that are promised to those who keep the Sabbath day holy. He said:
Oh, my dear brothers and sisters, there is power in keeping the Sabbath day holy—power to help others as well as ourselves. If we would have God’s blessings and protection as individuals, as families, as communities, and as nations, we must keep His Sabbath day holy.