http://thespiritmagnus.blogspot.com.es/
"The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin." - Catechism of the Catholic Church, par. 2181
In short: yes, it is a Mortal Sin ("grave sin") to miss Mass deliberately.
What is meant, though, by "deliberately"? In this context, "deliberately" is understood to mean a conscious decision to not attend Mass not due to circumstances that would otherwise prevent or make it difficult for a person to make it to Mass. For example:
- Making a conscious decision not to go to Mass due to being ill, incapacitated, caring an infant or an invalid would not be considered deliberately failing to meet the Sunday obligation due to being constrained by circumstances (if of course the individuals intentions would be to otherwise attend Mass) and therefore not a Mortal Sin;
- Making a conscious decision not to go to Mass out of disobedience or refusal to uphold the Sunday obligation and tenet of the Church would constitute Mortal Sin, i.e. the individual says in some manner of form, "I don't want to go!"
(...)
The experience made me think of the the capital vice ("deadly sin(s)") of sloth. The sense of laziness in itself is not sinful but is an inclination to act sinfully. In the instance of going to Mass, I'm sure we've all felt like we don't want to go to Mass and that the thought of giving yourself one more hour of leisure or rest, but there's an old adage that comes to mind: "Evil triumphs when the good men of the world do nothing" and how true does this ring for us as Christians?
Consider the good we miss out on in ignoring the Eucharist, the source and summit of Christian life (CCC, par. 1324). If every Catholic truly knew and understood the Eucharist, why the heck would you want to miss out on it? Furthermore, if every Christian knew and understood the Eucharist, they'd be flocking in the thousands to get in on some of that... Catholic stuff (*wink*).
Amen.