After the Resurrection the first day of the week was commemorated by the Apostles as the Lord's day because Sunday was the day on which Jesus conquered sin and death. Several weeks after that first Easter, the Holy Spirit descended on the Apostles. The first Christians had their meetings on Sundays. This began the tradition that continues through today. Together we give worship to God once a week on Sundays through our participation in the sacrifice of the Mass.
The Church obliges all Catholics to take part in the Sunday Mass each week unless there is a serious reason to excuse us. Why, does the Church do this? The answer is that it is good for us, for our spiritual health. In each and every Mass, Jesus becomes present on the altar in the very same Sacrifice of Calvary, though in an unbloody manner. We can scarcely appreciate the Gift that is before us upon that altar. He is there. He sees us; He hears us. It is our own imperfections and insensible dispositions that prevent us from knowing the beauty and glory of each Mass.
We have serious obligations to participate in Mass on other days that commemorate the significant events of our salvation. These are the Holy Days, namely, the Solemnity of Mary the Mother of God on January 1, Ascension Thursday, the Assumption of Mary, All Saints Day, the Immaculate Conception, and Christmas. When we are at Mass on these days or on any day, it is as if an ocean of God's grace is being poured upon us. Not only do we receive this benefit, but we also fulfill the virtue of religion whereby we render to God the worship that is due Him.
I indicated above that there are reasons that excuse the obligation to go to Sunday and Holy Day Masses. Being sick is one reason. Also, taking care of someone who is sick, whereby leaving that person even for a short time would be inadvisable.
Also excused is a person who is weak or advanced in age and walking the distance to church can dangerous. God does not want us to put ourselves or anyone else at risk because of the precept of participating in Mass. There are other reasons too, such as one's work requirement, although employers should be sensitive to their employees religious needs. Common sense is generally the rule.
May we all see Sunday Mass not so much as an obligation, but as a gift that we are privileged to partake in. May our understanding lead us to desire the Mass not only on Sundays but also during the week.