The Sabbath is and always has been Saturday, the Seventh Day of the week. It is observed from sundown Friday to sundown Saturday.
Sunday, the first day of the week, is not the Sabbath. This is man’s tradition, not God’s. Jesus rebuked the Pharisees for doing the same thing:
For laying aside the commandment of God, ye hold the tradition of men, as the washing of pots and cups: and many other such like things ye do.
Mark 7:8,9,13
And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.
Making the word of God of none effect through your tradition, which ye have delivered: and many such like things do ye.
God did not change the Sabbath to Sunday. And He did not make it void or of non-importance. Nowhere in the Bible will you find this. Some false teachers will take certain texts out of context however and try and claim this. This is why it’s important to read the Bible for yourself and look at ALL the texts that talk about the Sabbath and weigh the evidence for yourself.
Jesus, speaking of a future time, mentioned the Sabbath:
But pray ye that your flight be not in the winter, neither on the sabbath day:
Matthew 24:20