There's all kinds. You just can't see it because to admit it would mean that you have to reject the philosophy upon which you've created religious views.
In Jesus time:
Joh 12:20 And there were certain Greeks among them that came up to worship at the feast:
Joh 12:21 The same came therefore to Philip, which was of Bethsaida of Galilee, and desired him, saying, Sir, we would see Jesus.
There was no clear defining day when Christians began to forego observing the feasts of the Lord. But there were events that sped up the process, namely anti-semitism and the later prominence of the Roman church.
We know from history that as late as 365 AD council of The Council of Laodicea had to tell gentile Christians not to observe one of the Lord's feasts, the weekly sabbath:
#29: Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema from Christ. (Percival Translation ).
Paul had may opportunities in his letters to enjoin such behavior on the gentiles, but he never does . He had a perfect opportunity, for example, in 1 Cor. 11 when discussing the matter of the Lords Suppert (not passover), and he never mentioned feast-keeping. Its hard to believe that the same gentiles who could not rightly keep the very simple Lords Supper managed to get all the detailed feast days of Moses perfectly right without the need for a word from Paul.
Sure they did because that was the normal custom for Christians. That's what they were taught from scripture, the "old testament".
Act 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the Scriptures daily, whether those things were so.
Act 17:12 Therefore many of them believed; also of honorable women which were Greeks, and of men, not a few.
Act 15:21 "For Moses from ancient generations has in every city those who preach him, since he is read in the synagogues every Sabbath."
I realize how you want to read the verses, but it simply says there were certain Greeks among those who came up to worship at the feast. It does not say these Greeks were worshipping at the feast, which is youre reading. (Which would have been inappropriate for uncircumcised Greeks to observe.)
It may simply mean that Greeks traveled with Jews who were coming to worship, and these Greeks sought out Jesus. You are hard pressed to find evidence of Greeks (unless they are proselytes to Judaism) observing these Mosaic codes. Even the proselytes were not permitted to observe certain feast, such as Passover. Besides, its not even clear these Greeks were believers in Jesus at this time. They may have just been curious. Note what is written earlier, Therefore the people, who were with Him when He called Lazarus out of his tomb and raised him from the dead, bore witness. For this reason the people also met Him, because they heard that He had done this sign
You can not build a very strong case on pure speculation, esp. when all the evidence is against you.