Sunday is the historic Sabbath Day, and Mormons believe (like all other Christians) that it is set aside by commandment from God as a holy day. The Bible tells us that God created the world in six days and that on the seventh day he rested. From the time of Adam and Eve, the Lord has given commandment to set aside our labors on the seventh day and to rest, worship, and praise God.
While many people either do not believe this is a commandment any more or simply choose not to live it (whether they just do not believe in God or because they want an extra day to play), members of The Church of Jesus Christ of Latter-day Saints are still taught that observing the Sabbath Day is a very important commandment. For Mormons, the Sabbath is a day of spiritual contemplation and rejuvenation. Each Sunday meeting is three hours long, and members are expected to attend all three hour-long meetings. In addition to attending Church meetings, though, Mormons are asked to refrain from normal daily activities which could detract from their focus on the Savior. For example, Mormons are taught to not work on Sunday if they can help it; they are also asked to not shop in Sunday because this is encouraging employers to have their employees work on Sunday. Exercise and manual labor is best reserved for the other six days of the work, as are activities such as swimming and other recreation.
You may wonder what Mormons are “allowed” to do on Sunday. The fact is that this commandment, just like all the others, is for our benefit, not the Lord’s. Anyone who has faithfully lived the commandment of keeping the Sabbath holy can tell you that a strength has come into his or her life. The soul needs rest and rejuvenation just like the body does. The mind also benefits from this day of rest. The time sent with family and in service to others is invaluable in this rejuvenation process.
Keeping the Sabbath Day holy is a matter of faith, but once you gain a testimony of it for yourself, it becomes a joy to live this commandment.