@slanzill, your have been given many answers, I will try one more.
Lets start with the whole idea of the Precepts of the Church. Here is what the catechism says
2041 The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the very necessary minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbor
Ignoring for the time being what the precepts are, the concept of having precepts such as these certainly makes sense. A rough analogy is marriage: the absolute minimum of what we must do for our spouse is to be faithful. If we do not at least do that, we can certainly say our marriage is not going to go too smoothly. Well, as Christians, we all want to follow Christs’ top two commandmanets: love of God and love of neighbor. What is the minimum we must do to achieve this? Well, we could all come up with our own, but the Church answers this for us, it gives us a basic set of 5 laws we must follow. Not following these laws, not doing the minimum, says that we do not care about the love of God and neighbor, hence we are entering into mortal sin.
That is the answer as too why not following the precepts is a mortal sin, we are literally turning away from God, just as a husband who has an affair is literally turning away from his wife.
So the now lets move onto the first precept:
[2042] The first precept (“You shall attend Mass on Sundays and holy days of obligation and rest from servile labor”) requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days.
Again, this is rather clear. You seemingly have no problem with the requirement to sanctify Sundays and a holy day such as Christmas. But you consider it wrong to have to sanctify a feast day honoring Mary or the saints. For one thing, the selection of holy days is left to the national bishops because different parts of the world have different histories of what is important to them. For instance, in Mexico, the feast day of Our Lady of Guadelupe is immensely important. But that is likely not to be the case in a country such as England. So the different days in different countries again make sense. But why a Marian feast day? Why is some level of honor to Mary required for us to grow in love of God and neighbor? Because honoring Mary, praying for her intercession, etc are all part of the Catholic Faith. Some people may do it more than others, but there is a bare minimum. And failing to honor God’s mother to the minimum establish by the authority of the Church (is that in question?) is obviously telling God, we do not care in the least about your mother.