Sunday is the day set aside to honor God and should be spent differently from all other days. It is a day we raise our minds and hearts to God with deep reverence towards Him and with profound gratitude and prayer. In the Old Testament it was the Sabbath that was such a day but in the New Testament it is Sunday, the Lord's Day.
Moses was told, "Remember the sabbath day, to keep it holy. Six days shalt thou labor, and do all thy work: But the seventh day is the sabbath fo the Lord thy God: in it thou shalt not do any work" (Exodus 20:8-10). In the Old Testament the penalty for not keeping it holy was the death of the soul.
What were the reasons for this commandment from God?
a. It was hallowed by God in memory of the creation of the world. In Genesis ti says, "On the sixth day God ended His work which He had made; and He rested on the seventh day from all His work He had made. And God blessed the seventh day, and sanctified it: because in it hHe had rested from all His work which God created and made" (Gen 2:2-3).
b. The other reason is the remembrance of the liberation of the people of Israel from Egyptian bondage as we are instructed in Deut 5:12-15. Similarly the people of the New Testament were delivered from the bondage of sin by the death of Jesus Christ and it is prescribed also for us to consecrate the day of the Old Testament Sabbath on the day of the Resurrection, Sunday.
Metropolitan Gregory of St. Petersburg puts it this way: "The Lord has granted us six days of every week to carry out our business necessary for our earthly life, but the seventh day–only ne day–He appointed for rest under pain of eternal death for violating it..."
Saint John Chrysostom says: "It was the Lord's good will to prescribe that we dedicate one day in the weekly cycle to spiritual matters."
In the book of Acts we see that original Christians gathered on Sunday for the breaking of bread and listened to His teachings (Acts 20:7).
There are several obligation that Sunday imposes on us.
1. We should set aside all the business we need to engage in during the six days of the week to supporrt ourselves.
2. We should turn away from all impious acts that distract our souls from the remembrance of the Lord God, reverence towards Him, gratitude and a prayerful disposition of soul towards Him. This includes all unedifying reading, conversations and games where our soul might lose remembrance of God and potentially be carried away by delights and sin.
3. We are to attend the Divine Liturgy. This service is the ultimate remembrance of God's various blessings. In our attendance we reverence God, give Him thanks, and seek through our prayers that His blessings will continue to be given to us. We join in communion with Him as we partake of His body and Blood.
4. We should reflect on all of God's creation and His All-Powerful nature, His Wisdom, Goodness and unconditional Love for us. We should experience the wonder and awe of His creation in the natural environment. We should reflect on His life and the path He laid out for us through His death and Resurrection.
Metropolitan Gregory says. "You who love God: follow the path unto which the Lord has directed you and fear nothing...only try to please the Lord God, and then, remembering the words of the Holy Apostle, "If God be for us, who can be against us?" (Rom 8:31)... Do not be afraid, just try and avoid all occasions of sin through which our enemy always more easily lures us into his nets and ruins us."
You should contemplate His passion and death, how he suffered for us and think of what is written in the Gospel of John, "Behold how God so loved the world, that He gave His only begotten son, that whosoever believes in Him shall not perish, but have everlasting life." (John 3:16).
5. We should read the Scripture on this day just like the first members of His Church (Acts 20:7). We are all called to live a holy life in His image. "God has chosen us in Him before the foundation of the world, that we should be holy and without blame before Him." (Eph 1:4)
6. Metropolitan Gregory reminds us: "We must examine ourselves every day in relation to our salvation, so much more should it be our obligation on Sundays... Sunday before all other days should be a day on which we make the most attentive and detailed examination of our spiritual state in relation to salvation, and make a new, firm intention to root out from ourselves everything "opposed to God and our salvation."
Reference: How to Live a Spiritual Life, pp 112-137