Shavuot celebrates the giving of the Law of YHVH, the Kingdom to Israel, the ingathering and the Jubilee.
Really, there is no secret if you have the ability to see. Understanding of the covenant is simply hidden from the blind.
Yom Kippur has become a perverted Feast in the hands of religion. Judaism changed it, Christianity ignores it and Islam doesn’t even know about it. Kingdom citizens are required to know and do what YHVH said about it and to stick with only that.