That still doesn't invalidate my point. You're trying to say that the holy days were not taught because the holy days were not referenced in the letter. The ten commandments for the most part were also not mentioned in the letter. They were as much a part of the law, if not more so, then the holy days. By your logic, gentile converts weren't being taught to refrain from killing, worshipping false Gods or worshipping idols.
Your statement that the holy days were unique to Judaism is perhaps technically accurate, but biblically there are examples of non-Jews not only observing, but worshipping at God's Holy Days:
Joh 12:20 And there were certain Greeks among them that came up to worship at the feast:
Also, verse 21 is not even included in the letter
It doesn't matter. Verse 21 was an explanation of WHY they didn't include MORE in the letter:
Act 15:20 Instead, we should write a letter telling them to keep away from things polluted by false gods, from sexual sins, from eating the meat of strangled animals, and from eating bloody meat.
Act 15:21 After all, Moses' words have been spread to every city for generations. His teachings are read in synagogues on every day of worship.
For generation people had been learning about the Holy Days. This was the normative cultural and scriptural thing to do. It was commanded by God in the only holy scriptures they had. Of course they were observing them. The only thing that makes you think they weren't is that your are looking backward under the prism of your current beliefs and current tradition.