What are we commanded to do concerning the Lord's Supper?
Key Verse:  For as often as ye eat this bread, and drink this cup, ye do show the Lord's death till he come.  [1 Corinthians 11:26]
Origins of the Lord's Supper

The Lord's supper was instituted on the night of the observance of passover, the annual observance of when the death angel passed over Egypt.

And when the hour was come, he sat down, and the twelve apostles with him.  And he said unto them, With desire I have desired to eat this passover with you before I suffer:  For I say unto you, I will not any more eat thereof, until it be fulfilled in the kingdom of God.  [Luke 22:14-16]

After the passover supper was eaten, our Lord instituted the Lord's supper.

And he took the cup, and gave thanks, and said, Take this, and divide it among yourselves:  For I say unto you, I will not drink of the fruit of the vine, until the kingdom of God shall come.  And he took bread, and gave thanks, and brake it, and gave unto them, saying, This is my body which is given for you: this do in remembrance of me.  Likewise also the cup after supper, saying, This cup is the new testament in my blood, which is shed for you.  [Luke 22:17-20]

The Lord's supper is to be a perpetual statute until fulfilled in the kingdom of God.  Jesus Christ became the passover and was slain the same day that the death angel passed over Egypt.  The day the passover originally occurred and was observed annually was the fourteenth day of the first month of the Hebrew year. 

In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even.  [Exodus 12:18]
The example that Jesus set forth is the one we follow, once a year on the fourteenth day of the first month.  
Necessity of the Lord's Supper

Jesus made it very plain that the observance of the Lord's supper is necessary for salvation.

Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.  Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.  [John 6:53-54]

We are told that we must purge out the leaven of sin before partaking of the Lord's Supper.  The act of partaking of the emblems of the Lord's Supper by themselves does nothing.  We must do our best to keep God's commandments and follow Christ's example for the Lord's Supper to have meaning.

Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?  Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:  Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.  [1 Corinthians 5:6-8]

Procedure

Before we observe the Lord's Supper, we must first make ourselves ready to partake of the emblems of the body and blood of our Saviour.  The process of examining ourselves is very important, because if we partake of the body and blood of Jesus unworthily, it will have detrimental affects on us.
Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord.  But let a man examine himself, and so let him eat of that bread, and drink of that cup.  For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.  For this cause many are weak and sickly among you, and many sleep.  For if we would judge ourselves, we should not be judged.  [I Corinthians 11:27-31]
As part of the process of making ourselves ready to partake of the emblems, we follow the example of Jesus by humbling ourselves, and we wash one another's feet.  This is done by girding ourselves with a towel, pairing off and washing each others feet.  Jesus told us to follow His example and do this.
After that he poureth water into a basin, and began to wash the disciples' feet, and to wipe them with the towel wherewith he was girded. Then cometh he to Simon Peter: and Peter saith unto him, Lord, dost thou wash my feet?  Jesus answered and said unto him, What I do thou knowest not now; but thou shalt know hereafter.  Peter saith unto him, Thou shalt never wash my feet. Jesus answered him, If I wash thee not, thou hast no part with me.  Simon Peter saith unto him, Lord, not my feet only, but also my hands and my head.  Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all.  For he knew who should betray him; therefore said he, Ye are not all clean.  So after he had washed their feet, and had taken his garments, and was set down again, he said unto them, Know ye what I have done to you?  Ye call me Master and Lord: and ye say well; for so I am.  If I then, your Lord and Master, have washed your feet; ye also ought to wash one another's feet.  For I have given you an example, that ye should do as I have done to you.  [John 13:5-15]
After the footwashing, we partake of the emblems which represent the body and blood of Jesus.  First, the unleavened bread is taken and prayed over. Then it is broken and divided among the participants.  The bread is unleavened to represent the sinless body of Jesus.  Leavening represents sin.
Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.  [I Corinthians 5:8]
... Jesus took bread, and blessed it, and brake it, and gave it to the disciples, and said, Take, eat; this is my body.  [Matthew 26:26]
Next, He took the grape juice, the fruit of the vine, blessed it and gave it to His disciples.  This represented Jesus's sinless blood, so it must have no leavening in it.  That is why it must be nonalcholic.  
And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it;  For this is my blood of the new testament, which is shed for many for the remission of sins.  [Matthew 26: 27-28]
We keep this as Jesus commanded us to keep it.  

Conclusion

Our Lord Jesus Christ has set us an example.  The Lord's Supper is to be observed until Christ returns to fulfill the kingdom of God.  It is to be observed annually, at the beginning of the fourteenth day of the first month, Abib, in the evening.  The new moon nearest the vernal equinox determines the first day of the month Abib.  Followers of Christ are to partake of the emblems of the Lord's Supper on the fourteenth day of this month after examining themselves to insure they are worthy.

The Church of God (7th Day) publishes the date that corresponds to the fourteenth day of the first month, Abib, each year in the January edition of the Advocate of Truth.