My wife and I attend mass just about every day. We had intended to go to mass early, due to a schedule meeting we didn’t go knowing that we could attend the evening mass We do charity work and were involved in collecting diapers from different points in the city. We finished late and were on the way home, because it was late I said whey don’t we stop for dinner. During dinner it I looked at my watch and realized that we had forgotten mass. Is this a mortal sin?
Sins are things we choose to do or not do. Being forgetful is not itself a sin. The only way I see a sin in your situation is if you forgot about attending Mass because you placed very little importance on attending. But that doesn’t appear to be the case in your situation.