Q. 1. If I miss going to Holy Mass on Sunday, can I make it up on another day during the week?
A. 1. Sunday is a day of obligation. "You shall attend Mass on Sundays and on holy days of obligation and rest from servile labour." To avoid going to Mass on Sunday without a valid excuse, such as sickness, it means one is committing a serious sin. (A serious sin was called a "mortal sin" prior to Vatican II.)
Having missed Holy Mass on Sunday, one is required to confess this sin prior to receiving Holy Communion at the next Mass attendance.