Just because He calls them your Sabbaths, or your feast days, does not necessarily mean that they were not even meeting on the correct days. The context shows that they were doing the ceremonial things (Isaiah 1:11, 13; Amos 5:22).
If Israel regarded the Sabbath as merely ceremonial, then they were at least keeping the Sabbath in a ceremonial way. When He says "your new moons," or "your Sabbaths," they could very well have been on the same days that God commanded, and not something "new" that they came up with. There is a possibility in the book of Amos, because of Jeroboam I, that they indeed may have been keeping different days. But, in Isaiah 1, it seems "the Sabbaths" He refers to there, are "the Sabbaths" that we know of today as Saturday.
Thus, if indeed they were still keeping the weekly Sabbaths and the holy days (at least in terms of the right days on the calendar), then God's displeasure was caused by the way that they were keeping them, their attitude and lack of understanding as to why they should be keeping them. That is what concerned God. So bad were these issues, that as far as God was concerned, those days that they were keeping were no longer His, and He was separating Himself from them.
For short periods of time, small groups of people in Israel kept it right—but how to keep it was almost always a bone of contention between God and Israel. That issue is written about frequently in the Bible. It is not that they were keeping the wrong days, but how they were keeping them and their lack of understanding as to why they were keeping them that God was concerned about.
There is a great deal in the Bible about this commandment. When one includes what is written concerning keeping the holy days, the annual Sabbaths, with what is written concerning keeping the weekly Sabbath, there is more written directly about this commandment than any other, except the first commandment. We are not without instruction as to God's mind toward it—far from it. We have a great deal of instruction on how we should keep the Sabbath.
It is well understood that God did not inspire a list of hundreds of dos and don'ts to be written down. Instead, He chose to reveal by means of a few commands, examples, and broad principles, that we are supposed to study into, meditate upon, reach conclusions, and put them into practice in our lives. It was done this way to teach us to think through the process of choosing and coming to an understanding of why we are doing these things, developing our understanding of the mind of God.
The goal of this way is not that we would become creatures of rote, but rather, that we would do things because they are right and avoid other things because they are wrong. We would be making choices of our own free will that are in line with the mind and will of God.
The Sabbath has often been referred to as the "test commandment." God is testing the intention, the motivation that precedes the act and provides us with our justification, as well as what we will permit ourselves to do. Sometimes, in defending ourselves, we will say, "Well, I didn't mean to hurt you." Maybe not, but the fact is that the other person was hurt. This position is not good enough, because it still falls short of the glory of God. It is good to remember "the road to hell is paved with good intentions." God wants our intentions and our acts to be right. If we get the intention right, there is a far greater chance that the acts we permit ourselves to do on the Sabbath will be right. It must be this way, because the batting average for right intentions bringing forth right acts is exceedingly higher than the other way around. God wants us to understand why we are doing what we do before we do it.