There are several questions in here. Let’s sort through them:
[quote=Jeff_B]If one were to miss a Sunday mass, say, because of an obligation, work, or family related, and instead went to a Saturday mass instead, would that still be a mortal sin?
This is actually three questions: What constitutes a mortal sin? Is missing Sunday Mass because of a work or family obligation a mortal sin? Can Saturday Mass suffice for the Sunday obligation?
What constitutes a mortal sin?
Three conditions are required: grave matter, full knowledge of the gravity of the action, full and free consent to the action. If any of the conditions are missing there is no mortal sin (cf. CCC 1857-1859).
Is missing Sunday Mass because of a work or family obligation a mortal sin?
The Catechism of the Catholic Church (2181) states:
The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on the days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.
You will have to determine if the particular work or family obligation qualifies as a serious reason for which you are exempted from the Sunday obligation. If you need help with such discernment, you should discuss your circumstances with a balanced, orthodox confessor or spiritual director. If you reach the conclusion that the circumstances in question meet the requisite “serious reason,” you are exempted from the Sunday obligation.
Can Saturday Mass suffice for the Sunday obligation?
If by “Saturday Mass” you mean the daily Mass that is celebrated on Saturday, no. If you mean the vigil Mass that is celebrated on Saturday afternoon or evening in anticipation of Sunday, yes, the vigil Mass suffices for the Sunday obligation.
[quote=Jeff_B]Can one who might be employed over a weekend be able to attend a weekday mass and still fulfill their obligation?
Weekday Masses do not substitute for the Sunday obligation. One must either attend the vigil Mass on Saturday, determine that one has the Catechism’s requisite “serious reason” for missing the Sunday Mass, or be “dispensed by [his] own pastor” (cf. CCC 2181, CIC 1245).