Sometimes we are misled by an honest translation. The interpreters who came out with the King James version, and also the New King James version and many others besides, chose to interpret a Greek word here as hypocrites. I am not saying that is wrong, but more modern translations say that there is an English word that better fits what they feel that Jesus meant. Although hypocrites is not entirely wrong, it does not give as good as a picture, they feel, of what He intended.
The word that they use is the word casuists; the scribes and the Pharisees were casuists. It is not a word that we normally use any more in the English language. A casuist is a person who has difficulty separating. Another way—to make it more clear—would be: a casuist is a person who strains at gnats and swallows camels. A casuist is a person who is rigid and inflexible. A casuist is a person who has to have a rule for everything. A casuist is a person who has very great difficulty with the spirit of the law. A casuist is a person who will argue incessantly over the etymology of a word, and he will miss the intent of what is said.
That is what Jesus was talking about here. These people were casuists—they missed the point. Were they were keeping the Feast? Yes, they were. But they rejected the commandments in the way that they did it. The Feast became to them a formalized, ritualistic system by which, if they adhered to exactly what the letter of the law said, then they could feel free and right before God.