These were the minimum requirements for gentiles to initially follow in order to get along with the jews. The early church was a co-mingling of Christian Jews and converted gentiles. Many of jews still had hangups about cermonial and traditional rules when it came to associating with gentiles. A good example of this is in Galatians chapter 2. Remember that many of these laws and traditions were manmade...not scriptural.
So the converted jews were insisting that in order to be SAVED one had to observe ALL the law of Moses:
Act 15:1 And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved.
Of course that's not true. One doesn't have to observe all the laws or Moses...or any of them to be saved. One only has to receive God's spirit, as the gentiles had.
BUT, there was still the issue of obedience to God's laws. Obedience to God's law comes as one learns and grows spiritually.
Please note that I am not advocating that all of the old testament laws still need to be followed. Paul makes it clear exactly which ones were obviated by Christ...primarily the laws pertaining to sacrifice and any and all functions related to the Levitical priesthood.
So the council at Jerusalem outlined the minimum requirements needed to preserve peace among the differing brethren...because after all they would learn the rest in the synagogue every sabbath.
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.
A further point about the holy days: I believe the holy days exist above and beyond any of the covenants. They are apart from any law, past or present. The proof of this is that they still exist in the future after Christ returns to earth:
Zec 14:16 And it shall come to pass, that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the LORD of hosts, and to keep the feast of tabernacles.
Zec 14:17 And it shall be, that whoso will not come up of all the families of the earth unto Jerusalem to worship the King, the LORD of hosts, even upon them shall be no rain