In order to properly observe those holy days, one would need to STRICTLY obey every injunction connected with them. And that cannot be done honestly by anyone today. As far as God is concerned, it must be all or nothing, since these are His commandments. Don't forget that Nadab and Abihu tried to circumvent those commandments and died and went to Hell.
THE FEASTS WERE FOR THE CHILDREN OF ISRAEL
Speak unto the children of Israel, and say unto them, Concerning the feasts of the LORD, which ye shall proclaim to be holy convocations, even these are my feasts. (Lev 23:2)
Why can no one observe those feasts today? Because there is NO temple in Jerusalem, NO tabernacle on earth, NO Levitical priesthood, NO Aaronic priests, and NO Levites.
THE LEVITICAL PRIESTHOOD WAS REQUIRED (INDEED ESSENTIAL)
But ye shall offer an offering made by fire unto the LORD seven days: in the seventh day is an holy convocation: ye shall do no servile work therein. (Lev 23:8) [BY THE PRIESTS]
Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest: And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it. (Lev 23:10,11)
And ye shall offer that day when ye wave the sheaf an he lamb without blemish of the first year for a burnt offering unto the LORD. (Lev 23:12) [BY THE PRIESTS]
And the priest shall wave them with the bread of the firstfruits for a wave offering before the LORD, with the two lambs: they shall be holy to the LORD for the priest. (Lev 23:20)
Those are just a few examples. I already told Blik to study Leviticus carefully (which she ignored). You can study chapter 23 and see how you (or she, or anyone else) will circumvent the temple and the Levitical priesthood while you observe those feasts. And unless you stick to the letter and the spirit of the Law, God will REJECT your Torah observance.
Furthermore, to observe these feasts is to TRAMPLE on the finished work of Christ. He already fulfilled the feasts of Passover, First Fruits, Pentecost, and the Day of Atonement. He already became the Whole Burnt Offering, the Meal Offering, the Sin Offering, the Trespass Offering, the Passover Lamb, and any other sacrifice recorded in the Law of Moses. That is why the Old Covenant has been ABOLISHED. (Not the Ten Commandments but the Old Covenant). So going back means rejecting Christ. A very serious matter.
And not as Moses, which put a vail over his face, that the children of Israel could not stedfastly look to the end of that which is abolished: But their minds were blinded: for until this day remaineth the same vail untaken away in the reading of the Old Testament [Covenant]; which vail is done away in Christ.