The obligation to participate in the Sacrifice of the Alter on Sundays and Holy Days is very grave and to fail in the fulfillment of this duty (Yes, Duty!) on these days, without a serious reason, is a mortal sin. Not only does the sinner thereby lose important graces, which he may never again receive, but God also punishes him severely, as has frequently happened.
The following are some of the many instances we might consider.

This happened near Rome. Three businessmen went to a fair at Cisterno, and after having transacted satisfactorily their business, two of them prepared to return home on Sunday morning. The third pointed out to them that they should attend Mass. They laughed at his words and replied that they could go to Mass some other day. So saying, they mounted their horses and set out on their return journey. The third man went to Mass and then proceeded to follow them. What was not his consternation on learning that both his friends had been killed on the road, victims of a dreadful accident!

Another awful punishment meted out by the Almighty to an unfortunate man in Rome itself. This man was a stone mason, and instead of attending Mass on Sundays, he worked publicly, thereby giving scandal. On the feast of Pentecost, he was engaged as usual at his sinful work on a high scaffolding when, behold, he was thrown to the ground and killed instantly.

St. Antoninus of Florence quotes another instance of untimely death as penalty for not attending Mass. Two young men went off together to hunt. One had been to Mass, but the other had not. A storm of thunder and lightning suddenly burst over them. One, the unfortunate man who had not gone to Mass, was struck dead by the lightning; whereas his companion escaped without a scratch.

"God is the same yesterday, today, and tomorrow!"

One of the principal duties of the Christian is to go to Mass on Sunday, the one day in the week consecrated to God.

The Mass obtains for us a happy death!

The crowning grace of our life is a holy and happy death. What avails it to have had a long and happy life, to have enjoyed all the comforts which earthly riches can give, all the honors the world can bestow, if in the end we die a bad death? An unhappy death means a never-ending eternity of misery and pain.

We can only die once, and if we die badly, there is no possibility of remedying the mistake. A bad death plunges a person into the fires of Hell forever and forever! It is consequently of the utmost importance that we do all in our power, that we use every means possible to secure a happy death.

Holy writers recommend various excellent methods whereby we may make our salvation certain, and all these we should use to the best of our ability. All agree, however, that the best and easiest of these means is the frequent assistance at Holy Mass.

Mgr. Nautier, Bishop of Breslau, notwithstanding his many duties and labors and grave responsibilities, sought to be present at as many Masses as he could which were celebrated in his Cathedral. At the moment of his death, his soul was seen mounting up to Heaven accompanied by many glorious Angels who sang sweet canticles of joy and praise.

A certain devout man had such confidence in the efficacy of the Mass that he did his utmost to be present at the Holy Sacrifice as often as he possibly could. He fell gravely ill and died with great peace and joy. His parish priest grieved much at the loss of this exemplary member of his flock and offered for his soul many sacrifices. Great was the surprise of the good priest when the dead man appeared to him radiant with  joy and thanked him for his charity, adding at the same time that he was in no need of prayers, owing to his frequent assistance at Mass, he was received immediately into Heaven!

God will never be outdone in generosity, and to be generous and faithful in attending Mass, to faithfully heed His Command, "Keep Holy The Lords Day, will be rewarded beyond our expectations.

Source: Fr. Paul O'Sullivan.