Leviticus 23:1-2 says, The Lord spoke to Moses , saying "Speak to the people of Israel and say to them, These are the appointed feasts of the Lord that you shall proclaim as holy convocations, they are my appointed feasts."
A Holy Convocation, the original Hebrew is kodesh mikraw. Kodesh is translated to holy, sacred, set apart, solem. Whatever this "convocation" is it must be important because it is set apart, not common, very important. Mikraw is translated to called out, public meeting, rehearsal. Rehearsal? A sacred rehearsal?
God appointed some holidays and they are to be a sacred rehearsal...what do you suppose that's all about?