I converted to the Catholic faith a few years ago. I'm still learning the Holy Day of Obligation calendar. This last week I didn't know it was a Holy Day of Obligation and worked right through the Assumption of Mary Mass. I learned of missing the mass after the fact. Is this a mortal sin I should confess before taking communion? I sometimes still get mixed up about what is considered a mortal sin and what is considered a venial sin and I'm completely confused about this one.