"...Attending Mass on Sunday is a solemn and binding obligation. If we deliberately fail in this matter, it is a grave Sin and we must go to Confession before receiving Communion..." FULL TEXT Letter from Archbishop

The Archbishop of St. Andrews in Edinburgh, Scotland has released a Pastoral Letter on the Feast of Corpus Christi. In it he explains that attending Sunday Mass for Catholics is necessary and that missing Sunday Mass, without a serious reason, is a grave sin. Missing Mass on Sundays deliberately must be confessed before receiving Communion.
Here is the FULL TEXT of the Letter from the Archdiocese Website: 
Edinburgh, Corpus Christi, 2018
My dear sisters and brothers in Christ,
Warmest greetings to you on the feast of Corpus Christi. I thought it would be helpful to write to you, especially on this day, to recall the central importance of the Sunday Eucharist as the very summit of the life of the local church. It is true that many people think of Sunday as just part of the weekend, a welcome break from the usual routine, and perhaps a day for family, or sporting activities and so on. Naturally, it's good to relax and make time for these things, but our culture has largely forgotten that Sunday is a weekly holiday because of its meaning as the Christian holy day. God commanded his chosen people to refrain from ordinary work for one day at the end the week. That "Sabbath", now our Saturday, reminded them that life has a higher goal than physical survival or financial gain. True fulfilment lies in relationship with our Creator and our hearts will always be restless until they rest in Him. So, the working week ended with everyone gathering in God's presence to offer sacrifices from the fruits of their labours and to receive renewed blessings through the hands of his priests. That weekly day of rest was an earthly reflection of God's own eternal restfulness. It was a constant commemoration of the Covenant between God and his people and also looked forward to a time when they would enter into his heavenly peace through the coming of the Messiah. As Christians we believe that we have found the Messiah. God has visited His people in the person of Jesus Christ, for He is God made flesh and blood. He is the at the centre of everything and everything finds its true meaning and purpose in Him. That eternal Day of peace and glory— the triumph of all that is good and just and holy, which the prophets all longed for—has already begun. It began on the first Easter Sunday when Jesus rose from the dead after offering his life in atonement for our sins on the cross. This is why Sunday, which is actually the first day of week, is now our Christian Sabbath, more properly called "The Lord's Day". St John Paul II wrote a wonderful letter to the whole Church on the importance of this day, in which he said that every Sunday "is Easter which returns week by week, celebrating Christ's victory over sin and death … It is the day which recalls in grateful adoration the world's first day and looks forward in active hope to the last day, when Christ will come in glory and all things will be made new". (Dies Domini,
1) And so: "Every seven days the Church celebrates the Easter mystery. This is a tradition going back to the Apostles, taking its origin from the actual day of Christ's Resurrection — a day thus appropriately designated 'the Lord's Day'." (Vatican II, Sacrosanctum Concilium, 6) When we gather for Mass on Sunday it is a foretaste of heaven. Because Jesus is truly present on our altar, all the angels and saints gather with us for the feast too (cf. Hebrews 12:22). We are caught up together in adoration of the Lamb of God, who sacrificed Himself to take our sins away and draw us into his divine life through communion with his own Body and Blood. Every Mass is actually that one same sacrifice offered throughout time and space. It is only this great Sacrifice that enables us to live authentically holy lives. It is only this Blessed Sacrament of his Body and Blood that empowers us to do real and lasting good in the world. Jesus Himself said that it is only if we are joined to Him in the Eucharist that we can hope to be saved for eternal life (cf. John 6:53). I will write to you again about these deeply important teachings; but, for now, I would like to re-emphasise something I wrote in my pastoral letter to you in 2015: "We belong to the Church because we belong to Jesus Christ, and we can only belong fully to Jesus Christ by being members of His Church". And we are only fully members of Christ and his Church when we faithfully join ourselves to the offering of his Sacrifice in the Mass every Sunday. It is this and this alone that makes us a community and binds us together as one family in the Church. If we cut ourselves off from this mystery of grace, we not only cut ourselves off from Him but from one another too. The community of God's people is impoverished by your absence. Of course, there can be unavoidable circumstances that prevent our coming to Mass; illness or caring for a sick child, for example; travelling abroad too, although in these days of the internet we ought to be able to plan ahead. But under normal circumstances, attending Mass on Sunday is a solemn and binding obligation. If we deliberately fail in this matter, it is a grave sin and we must go to confession before receiving communion again (cf. Catechism of the Catholic Church, 2181). So, I would like you to ask yourself today: is Jesus Christ in the Sacrifice of the Mass my priority every Sunday, or do other commitments take precedence sometimes? Coming to Mass only every other week, or occasionally, is not the same as being faithful to his New Covenant. Surely, we cannot treat Christ our Saviour as one option among others for us to shuffle at our convenience. Dear parents, you naturally want your children to achieve their full potential and find lasting happiness. There's no better thing you can do for them than to bring them to meet the living Lord Jesus at Mass every week. If you teach them like this to seek the Kingdom of God first, you can be sure He will bless them in all the other ways that you hope for (cf. Matthew 6:33). I know it isn't always easy in our secularised world. It may mean making sacrifices and standing up for your faith among friends or within families. But remember, Jesus already sacrificed everything for us, because He did not want anyone to be lost. At many times in the past, people have died for the Mass, and even today in some parts of the world our fellow Catholics risk their lives in order to get to Mass, because they believe that True Life depends on it. And they are right. Christ is our Life and the Mass is our lifeline. I therefore urge you, my dear brother and sisters, to make the choice to attend Sunday Mass, to make it a priority, and to bring others with you. Finally, rejoice in hope, be patient in difficulties, and be constant in prayer (cf. Romans 12:12), especially the Great Prayer which is the Sunday Eucharist, and the Lord will surely reward you. Invoking God’s abundant blessing upon you, and especially upon our children making their First Holy Communions at this time,
I am Yours sincerely in Christ,
 +Leo Cushley
Archbishop of St Andrews & Edinburgh
Source: http://www.archdiocese-edinburgh.com