If you miss Mass because you slept late, or you just had a lot to do that day and could not get up really early for say, an 8:00 A.M. Mass, is it a mortal sin and do you have to go to confession before you receive Communion again?
If you missed Mass because you accidentally overslept and there was no other Mass to which you could go that day, there would be no sin because there was no choice to skip Mass. In that case missing Mass was an accident.
Your second hypothetical situation is less clear. Are you talking about someone who accidentally missed Mass because he forgot to go early enough in the day, or are you talking about someone who decided that he had more important things to do than to go to Mass? As previously stated, accidentally missing Mass is not a sin. But unless the “other things” a person had to do on Sunday constituted a just reason to miss Mass (e.g., having to work at one’s job on Sunday) *and *the person had either forgotten to go to the Saturday vigil or for some other just reason could not go to the Saturday vigil, then deliberately choosing to put unjustifiable lesser duties ahead of going to Mass would be grave matter. If done with full knowledge and deliberate consent, it would be a mortal sin.
Forget Mass? Not a Mortal Sin by Karl Keating