@slanzill
I think people know what you’re trying to say, but you’re not making any sense and/or you’re approaching the question through a standpoint of fear. You’re also ignoring the fact that obeying our bishops is an opportunity to grow spiritually; so the Holy Days of Obligation are a positive commandment and we benefit from this when we carry through with it and obey the command.
There are certain days on the Liturgical Calendar that are especially important for public worship, such as Easter or Christmas or a few other days. Regionally, these days are defined as Holy Days of Obligation, such as the Feast of Our Lady of Guadalupe in Mexico.
Sin is almost always circumstantial. There are laws or rules or expectations made by governments or schools or workplaces and we obey these things. There are rules to follow on a bus. There are rules to follow in a library. There are rules to follow at a sporting event. There are house rules that are set up by people who live together. There are also obligations set up by the bishops and it is a sin against charity to disobey them. In the case of missing a Holy Day of Obligation, this is sufficient to be a grave matter, because we’re not assisting in the Mass that takes us back to the passion & sacrifice of Our Lord, and that is definitely not a small matter.