why do some christians insist that we should keep the saturday?

He did not abolish the OT. Without using religion as a Law to Justify and make firm law one particular day.

Is it not a good thing to set aside ONE day a week? In order to rest and reflect that there are higher things to consider. Like the importance of fellowship with your community, family and friends. Than our day to day drudgery and tendency to always work our fannies off? Pick a day and rest. Think on the benefits to your health and well being. Body and soul.

false teachings. they do not realize that Jesus is the Sabbath.

here is an interesting article in this regard.

http://www.gotquestions.org/Jesus-Sabbath.html

The thing which some Protestants just can't understand that Christianity originated from Judaism but Christianity is not Judaism it never was Judaism. The followers of the way where the Nazarene those who followed Jesus of Nazareth but the Nazarene were not Jews in fact the Jews thought of them as being heretics. The Nazarene believed that Jesus was the Messiah sent from the prophesy and they believed that the Father and the Son being God. There was not yet a Holy Trinity established but the Nazarene understood the Divinity of Jesus as God. Now the Nazarene even though they thought of themselves as being Jews had kept to the Mosaic Law and even observed Jewish Sabbath on Saturday. St. Paul came which created a schism of the Nazarene the ones who accepted the teachings of St. Paul would be called the Christians by the 2nd century the Christians were observing what would be called the Lord's Day which is celebration of the resurrection of Jesus being on Sunday the Christians based this upon the teachings of St. Paul who stated they were no longer under Mosaic Law therefore the Christians made their own traditions rather than to continue following the traditions of the Jews.
The Nazarene sect was around until the 4th century St. Jerome had documented the differences between the Nazarene and the Christians. He also distinguished them separately from the Ebionites who were deemed heretical by both the Nazarene and Christians because they rejected the Divinity of Jesus as God.