I think most people agree that Sabbath is on Saturday (from Friday sunset to Saturday sunset).

Why then go to church on Sunday?

Jesus observed Sabbath, and so did the early churches.

If ye love me, keep my commandments.
- John 14:15 (KJV)

Jesus also states clearly that He is "Lord of the Sabbath".

And he said unto them, That the Son of man is Lord also of the sabbath.
- Luke 6:5 (KJV)

Here's how Jesus summarizes the Ten Commandments.

Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.
- Matthew 22:37 (KJV)

The first four Commandments are to "love God", and the remaining six Commandments are to "love thy neighbor".

Please note that if you love your neighbor, in doing so, you also love God.

Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.
- Romans 13:10 (KJV)

It's quite clear from the above that we're meant to keep the Ten Commandments, and should therefore also keep Sabbath.

For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.
- James 2:10 (KJV)

We're even reminded to keep the Commandments in the book of Revelation, which is specifically meant for the end times.

Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
- Revelation 14:12 (KJV)

Let's look why the Sabbath day is so important to both God and us.

Remember the sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: 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: 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.
- Exodus 20:8-11 (KJV)

Note that this Commandment starts with "remember". This is because God knew we would forget. "Remember the sabbath" He says.

Here God asks us to rest (and not work, or disturb someone else's Sabbath) just as He did during the Creation. For us it's a way of saying "yes Father, I will work six days and then rest, just LIKE You created in six days and rested on the seventh. It's a confirmation that we have faith in our Father - even if we don't fully understand the mechanics of Creation).

God further says that He will hallow this day, which means that on this day God will draw close, and show you many things (Bible insights, understanding of prophesies, etc). God will reveal Himself to you, and in doing so, we get a better understanding of who God is. This relationship is important. God is not just some distant concept. We're His children, and God wants a loving relationship with us.

The fourth Commandment is sometimes seen as a seal, e.g. it has the name "Lord", the title "Creator", and the territory "Heaven and Earth" that normally go with a seal.

The decision to keep the Commandments can only be made voluntarily ("in their forehead").

The antichrist has such a 'seal', except it's called a mark.

And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads.
- Revelation 13:16 (KJV)

The mark of authority, means that antichrist thinks they have the authority (over God) to change His Commandments. As well as changing the Sabbath into Sunday (their 'holy' day), they have also removed the "no idols" commandment, and split up another commandment to get back to ten again.

Howbeit in vain do they worship me, teaching for doctrines the commandments of men.
- Mark 7:7 (KJV)

Most false teachings of today (doctrines) come from the Roman Catholic Church. Common doctrines currently being taught are: hell, rapture, etc. They infiltrated most churches long ago.

And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues.
- Revelation 18:4 (KJV)

If you have the mark of the beast, you will undergo the full wrath of God in the form of the seven plagues (in the book of Revelation).

But it doesn't have to be that way. You can still "come out of her" today. ("her" refers to "the whore of Babylon", and "whore" means "church with false teachings").

For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears.
- 2 Timothy 4:3 (KJV)

You've possibly been exposed to false teachings for a long time (many centuries in some cases), and some false teachings are indeed very deceptive and misleading, e.g. close to the truth, but opposite in many ways.

If you want you can ask for the Holy Spirit (Spirit of Truth) to show you the Truth.

Sabbath is not just any day, it stands for a choice you make. It's like saying "Father I love you, and I will keep your Commandments, and I will keep Sabbath as a sign of my love for you."

If ye love me, keep my commandments.
- John 14:15 (KJV)