Here is the reason to faithfully attend mass every Sunday from a scriptural reference
Heb 10: (all emphasis mine) https://www.biblegateway.com/passage/?search=heb+10%3A19-31&version=RSVCE
19 Therefore, brethren, since we have confidence to enter the sanctuary by the blood of Jesus, 20 by the new and living way which he opened for us through the curtain, that is, through his flesh, 21 and since we have a great priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful; 24 and let us consider how to stir up one another to love and good works, 25 not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near. 26 For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a fearful prospect of judgment,and a fury of fire which will consume the adversaries. 28 A man who has violated the law of Moses dies without mercy at the testimony of two or three witnesses. 29 How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace?30 For we know him who said, “Vengeance is mine, I will repay.” And again, “The Lord will judge his people.” 31 It is a fearful thing to fall into the hands of the living God."
deliberate Failure to meet on the Day, is already a sin with huge consequences
1.“the Day” = the LORD’S DAY / Sunday/ the Day Our Lord Resurrected
2.“sacrifice for sin”, & “blood of the covenant” = the words Our Lord spoke instituting the
Matthew 26:28 , https://www.biblegateway.com/passage/?search=Mt+26%3A28&version=RSVCE
Mark 14:24 , https://www.biblegateway.com/passage/?search=Mark+14:24&version=RSVCE
i.e. They are celebrating the Mass when they meet
THAT’s why those who deliberately fail to celebrate Mass (the Eucharist) on Sunday after being given the knowledge of truth, it says
◦ there no longer remains for Them, a sacrifice for sin
◦ They Spurn the Son of God
◦ They outrage the spirit of grace
◦ a fearful prospect of judgement awaits Them
◦ and a fury of fire will consume these adversaries
Does that sound like it’s only a suggestion to attend Mass on Sunday, or a command?
Does it sound like a venial sin to deliberately miss Mass on Sunday or a mortal sin?
One only has to look at the consequences mentioned. Those consequences DESCRIBE HELL for one who dies in mortal sin
From the CCC http://www.scborromeo.org/ccc/para/2178.htm