The following post is from Fr. Z's blog and is located at the link here. This is a concise explanation for understanding the obligation to attend Mass and when the obligation doesn't apply.
If one cannot get to a live Mass (illness of spouse, having to ride bike long distance to get to church, etc.), does attending a Mass on the internet fulfill the Sunday Mass obligation?
We have looked at this question quite a few times in the past, but it bears repeating for newcomers.
This is something of practical value that parish priests should teach to their flocks. When people have been made aware of obligations, they are – in my experience – sincerely interested in fulfilling them, provided they understand the “why” behind the obligation. At the same time, people also need to know enough about those obligations and the law so that they can be at ease about how to fulfill them and when they don’t. They need to know enough law so that they aren’t filled with anxiety or fear about their responsibilities.
If you cannot go to Mass, truly cannot, then the obligation is suspended.
If you can go, you go. If you can’t you can’t. God doesn’t ask the impossible.
If you are sick, you don’t have to fulfill the obligation. If you are old and afraid to go out alone, or that you might slip on the ice, you don’t have to fulfill the obligation. If you are far from a church while travelling and don’t know where to go or can’t get to a church, you don’t have to fulfill the obligation. If you are taking care of a sick person and cannot leave, you are not obliged to go to Mass.
Of course, if a person really can go to Mass, and doesn’t… well… don’t get hit by a truck, because you have probably committed a mortal sin, if you knew that not going was wrong, knew you could, and simply blew it off.
Furthermore, because it always comes up, watching Mass on the internet or on the TV does NOT fulfill the obligation. Doing so can be edifying (depending on the Mass, of course) and even consoling, but internet/TV Masses don’t fulfill the obligation.
Finally, the 1983 Code of Canon Law, in can. 1245 gives to pastors (in England “the parish priest”) the ability to grant a dispensation from the obligation in individual cases or else to commute the obligation to other pious works.
You can debate whether or not watching Mass on TV or the internet counts as a “pious work”.
Fulling our Mass obligation is a serious matter for our spiritual well being. That said, Holy Church’s laws underscore her practical experience of centuries, her common sense mercy, and her concern that we be at ease about how to fulfill those obligations.