So this is going to take awhile. So bear with me. These Holy Days were not only kept by the apostles, they were prophesised to be observed after Jesus's return.
May I remind you that nowhere in the Bible does it say that the Holy Days were only for the Jews. It does, however, say that these were God's festivals. His, not the Jews.
Lev 23:1 And the Lord spoke to Moses, saying, 2 “Speak to the children of Israel, and say to them: ‘The feasts of the Lord, which you shall proclaim to be holy convocations, these are My feasts.
Passover and the Days of Unleavened Bread: As you probably already know, this festival was in part a prophecy of Jesus's sacrifice. Jesus kept it as well.
Luke 22:14 When the hour had come, He sat down, and the twelve apostles with Him. 15 Then He said to them, “With fervent desire I have desired to eat this Passover with you before I suffer; 16 for I say to you, I will no longer eat of it until it is fulfilled in the kingdom of God.”
Not only did Jesus observe Passover, but He told the apostles how to observe Passover from now on.
19 And He took bread, gave thanks and broke it, and gave it to them, saying, “This is My body which is given for you; do this in remembrance of Me.” 20 Likewise He also took the cup after supper, saying, “This cup is the new covenant in My blood, which is shed for you.
And then the apostles gave us instructions on how to observe Passover. We weren't supposed to keep it as a drunken party or as a reason to focus on filling your belly. Nor was it a time to show off your accomplishments, your wealth, or a time to judge others. You were instructed to keep it as a solemn observance and as a time of year to examine yourself.
1 Corinthians 11:17 Now in giving these instructions I do not praise you, since you come together not for the better but for the worse. 18 For first of all, when you come together as a church, I hear that there are divisions among you, and in part I believe it. 19 For there must also be factions among you, that those who are approved may be recognized among you. 20 Therefore when you come together in one place, it is not to eat the Lord’s Supper. 21 For in eating, each one takes his own supper ahead of others; and one is hungry and another is drunk. 22 What! Do you not have houses to eat and drink in? Or do you despise the church of God and shame those who have nothing? What shall I say to you? Shall I praise you in this? I do not praise you. 23 For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; 24 and when He had given thanks, He broke it and said, 4“Take, eat; this is My body which is broken for you; do this in remembrance of Me.” 25 In the same manner He also took the cup after supper, saying, “This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me.” 26 For as often as you eat this bread and drink this cup, you proclaim the Lord’s death till He comes. 27 Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. 28 But let a man examine himself, and so let him eat of the bread and drink of the cup. 29 For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord’s body. 30 For this reason many are weak and sick among you, and many sleep. 31 For if we would judge ourselves, we would not be judged. 32 But when we are judged, we are chastened by the Lord, that we may not be condemned with the world. 33 Therefore, my brethren, when you come together to eat, wait for one another. 34 But if anyone is hungry, let him eat at home, lest you come together for judgment. And the rest I will set in order when I come.
1 Corinthians 5:6 Your glorying is not good. Do you not know that a little leaven leavens the whole lump? 7 Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. 8 Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.
So that's Passover. The apostles not only kept it, but even instructed us on how to follow it. Don't forget that much of this happened decades after Jesus's death AND most of these followers were Gentiles, not Jews.