Well here's Hebrews 8:8:
Heb 8:8 Because finding fault with them, He says: "BEHOLD, THE DAYS ARE COMING, SAYS THE LORD, WHEN I WILL MAKE A NEW COVENANT WITH THE HOUSE OF ISRAEL AND WITH THE HOUSE OF JUDAH—
God doesn't say he's going to make a covenant with gentiles...but only with the house of Israel and house of Judah.
Stay with me though because you're right in a way:
Speaking to former gentiles Paul wrote:
Eph 2:11 Therefore remember that you, once Gentiles in the flesh—who are called Uncircumcision by what is called the Circumcision made in the flesh by hands—
Eph 2:12 that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world.
Notice verse 12...gentiles are by nature not part of the commonwealth of Israel. To Israel applies EVERYTHING. Israel is where Christ is, where the covenant are.
Eph 2:13 But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.
The blood of Christ brings us INTO Israel and we then become part of the commonwealth....we CAN have Christ and can be part of the new covenant.
That's why the new covenant is only made with the houses of Israel and Judah and why gentiles can be included....it's because they become part of the house of Israel through belief in Christ.
Now all of this was just a really roundabout way to make the point that when scripture says:
Lev 23:2 "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.
When the Lord, in scripture, speaks to the children of Israel we would do well to keep in mind that it very well may apply to us.
Where does the word "only" appear in Hebrews 8:8? The new covenant is "for the Jews first then the Greek". The new covenant isn't just allowing gentiles into the old covenant. Nor does Ephesians even hint at that. It is an entirely NEW covenant, apart from the old covenant. Israel was always larger than and distinct from the Mosaic old covenant. "For they are not all Israel who are of Israel". Being a child of Israel of the old covenant does not grant entrance into the new covenant. The new covenant is for those who accept the Gospel, "I am not ashamed of the Gospel for it is the power of God unto salvation, to the Jew first then the Greek". The new covenant is apart from and distinct from the obsolete old covenant. "In speaking of a new covenant, he makes the first one obsolete. And what is becoming obsolete and growing old is ready to vanish away." It is "a better country." Which is why Paul specifically said Christians need no longer celebrate the festivals:
"Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath.These are a shadow of the things to come, but the substance belongs to Christ." (Col 2:16-17)