They did this in their ignorance and their impatience to get things moving. Even though most of the people wanted it, and a renowned religious figure proclaimed it "a feast to the LORD," it did not make it so. God was definitely not positively impressed, nor was Moses. In one of the gravest acts of presumption shown in God's Word, and one of the largest in terms of the number involved, they took it on themselves to add this to the worship of God. What they did was very seriously disrespectful to God; they attempted to configure the nature of God according to their own desires.
Proverbs 14:12 says, "There is a way that seems right to a man, but its end is the way of death." People say they keep Christmas and Easter to worship Christ, but they are also defining the nature of God according to their own ideas. Just as surely as the ancient Israelites blended paganism with what God truly revealed, so people do today. This is the basic principle of acts of presumption, and each of us has done this, not once, but sadly, repeatedly, even though we may know better.
Jesus says in John 4:24, "God is Spirit, and those who worship Him must worship in spirit and truth," meaning we must worship to the fullest of God's intent as revealed in His Word, with every act guided and determined by His revealed truth. Yet, how many corners do we cut when we feel it serves us better at the time?