A common misconception by Christians who are being introduced to Torah is mixing up the requirements for following the Law with what they see Jews do as culture. The Law, simply put, is legislated by G-d in Torah for us to follow. The requirement to follow the Law has never gone away, but true repentance and belief in Yeshua haMashiach (Jesus Christ) removes the consequences of sinning.
"Do not think that I came to abolish the Torah or the Prophets! I did not come to abolish, but to fulfill. Amen, I tell you, until heaven and earth pass away, not the smallest letter or serif shall ever pass away from the Torah until all things come to pass" (Matthew 5:17-18 TLV)
"Heaven and earth will pass away, but My words will never pass away." (Matthew 24:35 TLV, Yeshua is referencing His past words in chapter 5 and Isaiah 40:8 & 55:9-11)
Many Christians are being called by G-d to start observing and celebrating Torah, but their conception of what Torah entails hinders their obedience. Let's take the example of Shavuot, or the Feast of Weeks, called Pentecost in Greek.
The commandment for Shavuot is found in Leviticus 23:15-21. Simplified, it describes counting seven weeks after the end of Passover until the day of Shavuot. Shavuot is a harvest feast, and multiple types of offerings are listed to be given. Besides the offerings, the only instruction that is given is this:
"You are to make a proclamation on the same day that there is to be a holy convocation, and you should do no regular work. This is a statute forever in all your dwellings throughout your generations." (v. 21 TLV)
Besides giving an offering, the only stipulated requirement by G-d is to have a day of rest with no regular work. Like a Shabbat rest, we refer to this as a Yom Tov (lit. good day). Notice the "forever" in the wording. G-d never put an expiry date on His Law.
Then there's the Jewish culture that attaches meaningful actions to the holiday. For example, Shavuot is same day that Moses returned down from Sinai with the Torah. Legend has it that Mount Sinai erupted with flowers the same day when Moses returned, heralding the bringing of the Law. So, Jews often decorate their houses and synagogues with flowers and greenery.
The holiday is also celebrated with mainly dairy foods, to represent the "land of milk and honey." My family has cheesecake every year on Shavuot to celebrate. Some families have their men stay up all night reading Torah. My family had a competition to see who got farther before they fell asleep.
All of that culture is wonderful and is done in the heart of praising and honoring G-d,. However, we must be careful that no cultural tradition surpasses the rule of the Law. For example, if one family decides that their Shavuot tradition will be selling ice cream on the side of the road, that would be in violation of G-d's Torah. Commerce is a type of regular work, and that expressly goes against Torah. It doesn't matter if it was done with good intentions, it violates Torah. Most traditions, and the ones that have passed the test of time, are not in violation of Torah.
Often what happens is that a Christian thinks G-d is telling them to celebrate the feast and so they look up more information on how. They find the cultural traditions and are overwhelmed. They think G-d wants them to become full Jews, and they will descend down a doom spiral ending up with, "I'm not Jewish, I believe in Jesus!" Which is a sophism for another day.
Following Torah does not make one Jewish. Judaism is a religion and a culture, and to be Jewish, one must feel in their heart that they are Jewish as well. G-d is not calling everyone to become Jewish, otherwise how could He fulfill His own prophecies? Consider Zechariah 14:16-17
"Then all the survivors from all the nations that attacked Jerusalem will go up from year to year to worship the King, Adonai-Tzva’ot, and to celebrate Sukkot. Furthermore, if any of the nations on earth do not go up to Jerusalem to worship the King, Adonai-Tzva’ot, they will have no rain." (TLV)
G-d is saying that the nations that attacked Jerusalem (which is most of them) will have to go and celebrate the pilgrimage feasts. "What? How can they celebrate the feasts if they're not Jewish?" They are G-d's Feasts that He gave in Torah, not Jewish feasts. Jews celebrate G-d's feasts, and so should Christians, and everyone else.
If celebrating the feasts and following Torah made one Jewish, then why does G-d still refer to them as separate nations? Wouldn't they become all Jews and lose their nationhood? They celebrate the feasts and follow Torah, the Law, but they retain their own culture (that doesn't violate Torah) thereby still being separate nations.
Good, pure, honest culture is not in opposition to the Law. G-d made many nations, each special in their own way, for His glory. The same way that people have different personalities and it praises G-d, so should different nations have their own culture. However, just as morality supersedes any personality traits, so should the Law come before any culture.