What's the Deal with Holy Days of Obligation? Isn't Sunday Enough Already?

First, all of you, yes ALL of you, are planning on attending Mass this Friday (August 15) for the Feast of the Assumption of the Blessed Virgin Mary, right? Remember, it is a Holy Day of Obligation (one of the very few left in the United States) which means it is a mortal sin to deliberately miss Mass Friday. The Catechism reminds us:

The Sunday celebration of the Lord's Day and his Eucharist is at the heart of the Church's life. "Sunday is the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost holy day of obligation in the universal Church."
"Also to be observed are the day of the Nativity of Our Lord Jesus Christ, the Epiphany, the Ascension of Christ, the feast of the Body and Blood of Christi, the feast of Mary the Mother of God, her Immaculate Conception, her Assumption (that's Friday), the feast of Saint Joseph, the feast of the Apostles Saints Peter and Paul, and the feast of All Saints." (2177)

And from the Code of Canon Law:

"Sunday . . . is to be observed as the foremost holy day of obligation in the universal Church" (CIC, can. 1246 § 1).  

"On Sundays and other holy days of obligation the faithful are bound to participate in the Mass" (CIC, can. 1247). 

"On Sundays and other holy days of obligation the faithful are bound . . . to abstain from those labors and business concerns which impede the worship to be rendered to God, the joy which is proper to the Lord's Day, or the proper relaxation of mind and body" (CIC, can. 1247).

Our obligation to attend Mass on Holy Days, including all Sundays, is deadly serious, deadly for our souls if we freely choose to miss Mass and blow off the obligation. The Lord made this point clear centuries ago to His people Israel, by literally prescribing physical death for those who deliberately missed the Sabbath (the "holy day of obligation" of the Old Covenant),

And it came to pass, when the children of Israel were in the wilderness, and had found a man gathering sticks on the sabbath day, That they brought him to Moses and Aaron and the whole multitude. And they put him into prison, not knowing what they should do with him. And the Lord said to Moses: Let that man die, let all the multitude stone him without the camp. And when they had brought him out, they stoned him, and he died as the Lord had commanded. (Numbers 12:32-36) 

I can hear the atheist objection already,

"why is God soooooo mean? Isn't he just a bloodthirsty tyrant!? Look, He's demanding people be executed for gathering sticks on the Sabbath!! How can you Christians even take this seriously? etc. etc. etc. etc." 

Of course, this is reading the above completely out of context of the whole Bible (which is only correctly interpreted through the Church and by the New Testament), but leave that to a side. We might rightly ask, especially when reflecting on the teaching of Jesus, what on Earth is God doing here? Why is He laying down such a serious sentence on this crime?

UPDATE:
Unfortunately, the original ending of this post was deleted. Here is a brief ending getting at the same basic point. For more on the importance of setting aside Holy Days for the Lord, I recommend St. John Paul II's Apostolic Letter Dies Domini.

What is God doing? Why does He command the death of those who break His command to rest and worship on the Sabbath? To answer these questions one needs to remember to always read the Bible (especially difficult passages like the above) in context - in light of the New Testament, understood through the teaching authority of the Catholic Church, and in light of the historical and cultural times in which the passage was written. God here is dealing with the roughest of peoples. A semi-barabaric, nomadic people nation living in the bronze age. These people had not the benefit of the revelation of Jesus Christ or even of the later prophets or wisdom literature. They were fresh from worshipping Egyptian idols and frequently found having faith in God difficult despite His clear presence among them (think of the constant flow of miracles these men were witness to!). In this age the death penalty, indeed even stoning, was a common prescription for any serious offense. The message here is that it is deadly serious to not worship God. This is an important lesson for God to get across to His people because failing to offer God the worship He is due, while not affecting God in any way, is fatal to our souls - i.e. people can and do go to hell for eternity for refusing to order their lives to the ultimate reality - God. This isn't because God is angry at them (again our worship does nothing for God). Rather it is the necessary end result of disordering our lives. God so desires for His people to be united to Him in glory, that He is willing to stoop down and teach them in the only way they will understand - through punishing violations of the Sabbath rest as a capital crime.  The lesson for us, of course, is not to go around stoning people who refuse to head to church this Sunday (cf. Jn 8:7), but to understand just how serious the repercussions of ordering our lives away from God can be. In the end we all worship something - we all have a God we serve. It may not be a personal God - it may be money, power, sex, pleasure, etc, but we all worship something. If what we worship isn't the true God, we are on a sure path to hell. Stopping from the everyday hustle of the world and focusing on God on Holy Days (like this Friday) is an important way to avoid a fate far worse than that which the men gathering sticks met with.