Attendance at Mass

The Book of Hebrews chapter 10 tells us why we should attend Mass, the Eucharistic Liturgy.

Hebrews 10:3-29  (Explanatory comments added in parenthesis.  The reference to the ceremonial laws of the Old Covenant are colored in red, and the aspects of the New Covenant are in blue Scripture is in bold black.)

3 But in those sacrifices there is only a yearly remembrance of sins, 
4 for it is impossible that 
the blood of bulls and goats (according to the ceremonial laws)   take away sins. 
5 For this reason, when he  (Jesus Christ)  came into the world, he said: 
"Sacrifice and offering you did not desire, 
but a
 body  (holy and sanctified) you prepared for me; 
6  holocausts and sin offerings you took no delight in. 
7 Then I said, ‘As is written of me in the scroll, 
I come to do your will  (infused righteousness), O God.’" 
8 First he says, 
"Sacrifices and offerings, holocausts and sin offerings, you neither desired nor delighted in." These are offered according to the law. 
9 Then he says,
 "Behold, I come to do your will." He takes away the first to establish the second  (the moral law). 
10 By this "will," we have been consecrated through the offering of the 
body of Jesus Christ once for all.

(Christ died only once.  His Life, Death, and Resurrection  is offered to our Heavenly Father in reparation for our sins.  God the Father, who created time, is not constricted by it like we are.  For Him there is no past or present, there is only the eternal now.  Everything is present to Him.  Jesus is offered to Him once for all time as it is continually present to the Father.   Our participation in the Mass makes present to us the Heavenly Liturgy of the Son being offered to the Father.  The Sacrifice of the Mass makes present this one Sacrifice of Jesus Christ - His Life, Death, and Resurrection -  into our time that which is perpetually offered to the Father.  It is not a repetition of the offering because you cannot repeat that which never ends. 
Cf.  Revelation 5:6  "Then I saw standing in the midst of the throne and the four living creatures and the elders, a Lamb that seemed to have been slain."   This perpetual offering was prophesied in  Malachi 1:11  "For from the rising of the sun, even to its setting, my name is great among the nations;  And everywhere they bring sacrifice to my name, and a pure offering;  For great is my name among the nations, says the LORD of hosts.  (This pure offering that is offered everywhere from the rising to the setting of the sun is the offering of this perfect Sacrifice of Jesus to the Father.  Jesus died on the cross, but His Sacrifice must be offered to the Father.  And this is always done on the Altar. 
Hebrews 13:10  "We have an altar from which those who serve the tent have no right to eat."   RSV  Those of "the tent" are the priest’s of the Old Covenant. 
1 Corinthians 5:7-8  "For our paschal lamb, Christ, has been sacrificed.   Therefore let us celebrate the feast…"  NAB)

…19 Therefore, brothers, since through the blood of Jesus (the Eucharist) we have confidence of entrance into the sanctuary 
20 by the new and living way he opened for us through the veil, that is,
 his flesh (the Eucharist), 
21 and since we have "a great priest over the house of God," 
22 let us approach with a sincere heart and in absolute trust, with our hearts sprinkled clean from an evil conscience and our bodies
 washed in pure water. (Baptism) 
23 Let us hold unwaveringly to our confession that gives us hope, for he who made the promise is trustworthy. 
24 We must consider how to rouse one another to love and good works. 
We should not stay away from our assembly (the Mass), as is the custom of some, but encourage one another, and this all the more as you see the day drawing near. 
26 If we sin deliberately after receiving knowledge of the truth, there no longer remains 
sacrifice  for sins

(The sacrifice mentioned here is the perpetual offering of the Sacrifice of Jesus’s Life Death and Resurrection which we offer at Mass.  We need to participate in the Mass so that we might avail ourselves of God’s grace.  This is the only understanding of the word "sacrifice" that fits the context.  It is the only "sacrifice for sins" which brings redemption and forgiveness.  And if we reject, or ignore it there is no other sacrifice that will take its place.)

27 but a fearful prospect of judgment and a flaming fire that is going to consume the adversaries. 
28 Anyone who rejects the law of Moses is put to death without pity on the testimony of two or three witnesses. 
29 Do you not think that a much worse punishment is due the one who has contempt for the Son of God, considers unclean the covenant-blood by which he was consecrated, and insults the spirit of grace? 
(In the RSV verse 29  reads  "…profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace?"    To profane - to consider unclean - means to make it ordinary as opposed to that which belongs in the Temple and is consecrated to God.  It is the Holy Spirit who changes the bread and wine into the Body and the Blood of Jesus Christ. )