Someone recently asked this question. It's one I asked myself in the past. We can only really understand how having something more important or more interesting to do on Sunday, rather than attending Mass could be a 'mortal' as opposed to a venial sin, when we understand the nature of the Eucharist.
Jesus told stories about those who refused to attend the Wedding Feast, and the man who managed to get in, but had not bothered to dress for the occasion- and they end up in the dark.
How can we choose the darkness over the light?
What on earth could be more important than the gift of the Bridegroom? The Bridegroom is the Christ of the third sorrowful mystery, flogged and crowned, His wrists bound; He is seated for a moment, ready to embrace the cross and pour Himself out for His bride.
When we recognise that, and that He did all this for love of us, we could never find anything else more important.
So, are all those who miss Mass on Sunday in a state of mortal sin?
The answer to that is no. A person has to KNOW that what they are doing is grave matter to be have personally committed mortal sin, but that does not change the fact that ignoring the Eternal Sacrifice in favour of something else is grave matter.
I have sympathy with those who do miss Mass. I missed Mass every fortnight for years as I attended church with my protestant husband. I was not sure if it was allowed to do this so I went and asked my parish priest at the time. He not only gave the go-ahead, but informed me his sisters did the same thing and what a great idea it was.
It was years and years later before I learned the Truth.
And now my husband is Catholic too
The Church has not changed Her teaching on the Mass. If you can get there-go.
Obviously there are genuine circumstances where this is not possible. Nurses have to work weekends, as do other essential staff, so there may be times when missing Mass is unavoidable.
I have missed Mass a lot recently through health problems. In those times the Deacon of our parish has brought Our Lord into my home so that I may receive him. What an honour.
I was also blessed to receive Holy Communion from the hospital chaplain when I was in over Easter with pregnancy complications.
I'll write more on this.