Every Sunday when we attend Mass we are actually at the re-presentation of Holy Thursday (Last Supper), Good Friday, and Easter Sunday. We celebrate God’s totally unconditional love for us in Jesus’ sacrifice on the Cross. Then, just like the Israelites of the Passover, we are called to Communion with our God as His adopted children to partake (actually eat) of the Supper of the Lamb (Eucharist.)
Yes, it is a grave (used to be known as mortal) sin to refuse our Lord’s invitation. It is a great mercy on His part to invite us, and we should accept. Additionally, accepting His invitation and doing as He said in John 6, Jesus not only accepts us into His family, but He also forgives our venial sins if we repent of them, and grants us sacramental grace to avoid them in the future, allowing us to become more like Him as time goes on.
As always, God gave us the commandment to Keep Holy the Sabbath, and as always, it is because it is a great blessing for us to do so.
My recommendation to you would be to find a church that either offers a Saturday vigil or a Sunday Mass at a time that you can attend, even if it is a 7:00 am Mass. It may be a sacrifice, but your blessings will be great.
If you cannot find a Mass that you can attend (it happens in rural areas), pray, pray, pray, and then make an appointment and speak to your pastor. Many times, God will bless you for your desire to do right either by providing a schedule change either at work, or even in the Mass schedule itself. No promises, just the hope of something that has happened to me a time or two. God bless you!
In Christ's peace and joy,
Robin L.