"It seems the feast days are a sign for Israel only"
This is true during the present age
There are some Gentiles of the professing "church" who think they need to keep these days, and some of them even try to predict the Lord's next intervention based upon them ... this behavior is off course as far as I am concerned
For the Lord's ecclesia no day is more important over another .... and the related law keeping is not to be practiced
Most of current day Israel still keep laws, but to no avail ..... the Lord does not recognize the practice at this time .... Israel for the most part is not saved and He does not give credit for any law keeping
Even the "messianic Jews" of Israel today are teaching that one must keep the law and this is a contradiction
When the Lord's millennial kingdom comes upon the earth, He will re-instutite the times and seasons for the restored kingdom of Israel .... and so will the Gentiles during the period be required to do the same
But, this re-instuted law keeping will not replace His requirement for salvation which will be the same as it is today .... these things will be to remind Israel of their past and to realize the difference between then and His own sacrifice for one's salvation