When was the sabbath given? (seventh day, not Sunday)

Gen 2:2 And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made.
Gen 2:3 And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.

Did the Jews keep the sabbath before the Ten Commandments were given?

It wasn't until Exodus 20 that commandments given. These verses are in Exodus 16, before ten commandments and they were keeping the sabbath then like God instituted in Genesis 2.

Exo 16:23 And he said unto them, This is that which the LORD hath said, To morrow is the rest of the holy sabbath unto the LORD: bake that which ye will bake to day, and seethe that ye will seethe; and that which remaineth over lay up for you to be kept until the morning.

Exo 16:25 And Moses said, Eat that to day; for to day is a sabbath unto the LORD: to day ye shall not find it in the field.

Exo 16:26 Six days ye shall gather it; but on the seventh day, which is the sabbath, in it there shall be none.

Exo 16:29 See, for that the LORD hath given you the sabbath, therefore he giveth you on the sixth day the bread of two days; abide ye every man in his place, let no man go out of his place on the seventh day.

What did God say about the sabbath then in Ten Commandments?

He said to "REMEMBER" it. (Capitals and bold mine for emphasis). And God reminded them that the sabbath was from the beginnning.

Exo 20:8 Remember the sabbath day, to keep it holy.
Exo 20:9 Six days shalt thou labour, and do all thy work:
Exo 20:10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
Exo 20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.

Does God care if we keep the sabbath?

Exo 31:13 Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it is a sign between me and you throughout your generations; that ye may know that I am the LORD that doth sanctify you.
Exo 31:14 Ye shall keep the sabbath therefore; for it is holy unto you: every one that defileth it shall surely be put to death: for whosoever doeth any work therein, that soul shall be cut off from among his people.
Exo 31:15 Six days may work be done; but in the seventh is the sabbath of rest, holy to the LORD: whosoever doeth any work in the sabbath day, he shall surely be put to death.
Exo 31:16 Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant.
Exo 31:17 It is a sign between me and the children of Israel for ever: for in six days the LORD made heaven and earth, and on the seventh day he rested, and was refreshed.
Exo 31:18 And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God

Deu 5:12 Keep the sabbath day to sanctify it, as the LORD thy God hath commanded thee.
Deu 5:13 Six days thou shalt labour, and do all thy work:
Deu 5:14 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, nor thy manservant, nor thy maidservant, nor thine ox, nor thine ass, nor any of thy cattle, nor thy stranger that is within thy gates; that thy manservant and thy maidservant may rest as well as thou. Deu 5:15 And remember that thou wast a servant in the land of Egypt, and that the LORD thy God brought thee out thence through a mighty hand and by a stretched out arm: therefore the LORD thy God commanded thee to keep the sabbath day.

How Long were the Children of Israel to Remember the Sabbath?

Exo 31:16 Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant.

Lev 24:8 Every sabbath he shall set it in order before the LORD continually, being taken from the children of Israel by an everlasting covenant.

Did Jesus' Disciples keep the Sabbath?

Luk 23:55 And the women also, which came with him from Galilee, followed after, and beheld the sepulchre, and how his body was laid. Luk 23:56 And they returned, and prepared spices and ointments; and rested the sabbath day according to the commandment.

Why were the disciples assembled on Sunday in Jerusalem during Pentecost instead of the Sabbath?

Jesus told them to wait in Jerusalem

Luk 24:49 And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high.

Pentecost was the firstfruit offering (Jesus was first fruit of resurrection)

Lev 23:9 And the LORD spake unto Moses, saying,
Lev 23:10 Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest:
Lev 23:11 And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it.
Lev 23:12 And ye shall offer that day when ye wave the sheaf an he lamb without blemish of the first year for a burnt offering unto the LORD.
Lev 23:13 And the meat offering thereof shall be two tenth deals of fine flour mingled with oil, an offering made by fire unto the LORD for a sweet savour: and the drink offering thereof shall be of wine, the fourth part of an hin.
Lev 23:14 And ye shall eat neither bread, nor parched corn, nor green ears, until the selfsame day that ye have brought an offering unto your God: it shall be a statute for ever throughout your generations in all your dwellings.
Lev 23:15 And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
Lev 23:16 Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD.

Pentecost and the Comforter was fulfillment of John's prophesy

Joh 1:33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.

Luk 3:16 John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:
Luk 3:17 Whose fan is in his hand, and he will throughly purge his floor, and will gather the wheat into his garner; but the chaff he will burn with fire unquenchable.

Joh 14:15 If ye love me, keep my commandments.
Joh 14:16 And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever;
Joh 14:17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
Joh 14:18 I will not leave you comfortless: I will come to you.
Joh 14:19 Yet a little while, and the world seeth me no more; but ye see me: because I live, ye shall live also.
Joh 14:20 At that day ye shall know that I am in my Father, and ye in me, and I in you.

Joh 15:26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:

Joh 16:7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.

1Co 15:20 But now is Christ risen from the dead, and become the firstfruits of them that slept.

Sabbath and the Tribulation

Mat 24:15 When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:)
Mat 24:16 Then let them which be in Judaea flee into the mountains:
Mat 24:17 Let him which is on the housetop not come down to take any thing out of his house:
Mat 24:18 Neither let him which is in the field return back to take his clothes.
Mat 24:19 And woe unto them that are with child, and to them that give suck in those days!
Mat 24:20 But pray ye that your flight be not in the winter, neither on the sabbath day:
Mat 24:21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
Mat 24:22 And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.

Mar 13:14 But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains:
Mar 13:15 And let him that is on the housetop not go down into the house, neither enter therein, to take any thing out of his house:
Mar 13:16 And let him that is in the field not turn back again for to take up his garment.
Mar 13:17 But woe to them that are with child, and to them that give suck in those days!
Mar 13:18 And pray ye that your flight be not in the winter.
Mar 13:19 For in those days shall be affliction, such as was not from the beginning of the creation which God created unto this time, neither shall be.
Mar 13:20 And except that the Lord had shortened those days, no flesh should be saved: but for the elect's sake, whom he hath chosen, he hath shortened the days.