Is it your position that the letter included anything and everything that the gentiles were to observe and anything else was up to them?
For example the letter mentions nothing about not killing, nothing about having false gods, nothing about hating your neighbor and nothing about greed. In fact, there are thousands of things NOT mentioned that any Christian would recognize as sinful behavior.
The answer of course is that this letter didn't occur in a vacuum. The gentiles WERE being taught to obey and observe the basic tenents of Judaism and early Christianty...in other words they were being taught about the sabbath, holy days and ten commandments. Thankfully God left us clear scriptural record that this was so:
Act 15:20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
Act 15:21 For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day.
See there...verse 21? Every sabbath the gentiles were learning about God's law in the synagogues.
You can't have it both ways on this. If you are saying that omission of specific instruction in the bible is permission to disregard the bible on God's holy days then it's also permission to disregard the bible in every other moral precept and law that God enumerated in the old testament.