Um, not exactly the demographic cited for: I would actually recommend James over Acts for anyone who has been a Christian for some length of time
As for Keep the Sabbath, and the appointed feasts these are not detailed as essential in Acts. Namely the letter in Acts 15 to gentile believers reads as follows:
The brothers, both the apostles and the elders, to the believers of Gentile origin in Antioch and Syria and Cilicia, greetings.
Since we have heard that certain persons who have gone out from us, though with no instructions from us, have said things to disturb you and have unsettled your minds, we have decided unanimously to choose representatives and send them to you, along with our beloved Barnabas and Paul, who have risked their lives for the sake of our Lord Jesus Christ. We have therefore sent Judas and Silas, who themselves will tell you the same things by word of mouth. For it has seemed good to the Holy Spirit and to us to impose on you no further burden than these essentials: that you abstain from what has been sacrificed to idols and from blood and from what is strangled and from fornication.
If you keep yourselves from these, you will do well. Farewell.
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Keeping the feasts and the Sabbath is what every believer in Acts was doing.
It was detailed as essential by Yeshua himself in two chapters in Matthew, and expressly in detail by John in his first epistle, and by Paul in Romans 2.