Lets look at Acts 20 in Context
Acts 20:5-12 (NKJV)
5 These men, going ahead, waited for us at Troas. 6 But we sailed away from Philippi after the Days of Unleavened Bread, and in five days joined them at Troas, where we stayed seven days.
Ministering at Troas
7 Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight. 8 There were many lamps in the upper room where they were gathered together. 9 And in a window sat a certain young man named Eutychus, who was sinking into a deep sleep. He was overcome by sleep; and as Paul continued speaking, he fell down from the third story and was taken up dead. 10 But Paul went down, fell on him, and embracing himsaid, “Do not trouble yourselves, for his life is in him.” 11 Now when he had come up, had broken bread and eaten, and talked a long while, even till daybreak, he departed. 12 And they brought the young man in alive, and they were not a little comforted.
The fact that they mentioned the Days of Unleavened Bread years after Jesus died shows they were still observing the Feast Days, so if they were still observing those, its more than likely a good chance they were observing the Sabbath as well.
Now lets address verse 7, Paul got together with the disciples on the 1st day of the week to bread break, but since he was ready to depart the next day, its clear this meeting was right after the sun went down Saturday since a new day started once the sun went down as to why his message lasted until midnight.
This makes much more sense since Paul speaking from 6-7pm-ish, when the sun went down until midnight is waaaay more believable than Paul meeting early on a Sunday morning and his his message being 12-15 hours until midnight.
So Paul met right after the sun went down Saturday night, which is now Sunday(first day of the week), spoke with the disciples until midnight (message until midnight), and departed on Sunday morning (ready to depart the next day).
Acts chapter 15 no where released the gentiles from anything, it just gives them some basics things to refrain from to get started and they will learn more as they go along. No different than learning more as you go along at your job when you just begin.
Acts 15:21 (NKJV)
21 For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath.”
This is another clear indication that the Sabbath was still being kept and Acts 20 shows it was still being kept was well, since they met right after the Sabbath was over.