There are over 300 predictions about the Messiah in the Old Testament. After the Resurrection Jesus accompanied two disciples on their journey and "explained to them the things concerning Himself in all the Scriptures" (Lk 24:27). Later he reminded his disciples that, "'These are My words which I spoke to you while I was still with you, that all things which are written about Me in the Law of Moses and the Prophets and the Psalms must be fulfilled'" (:44). It is God's word reinforced by Isaiah saying "'truly I have spoken'" (Isa 46:11). It is God '"declaring the end from the beginning'" (:10). Isaiah "moved by the Holy Spirit spoke from God" (2Pe 1:21). "All scripture is inspired by God" (1Ti 3:16). Peter explains that "we have the prophetic word made more sure" (2Pe 1:19). Prophecy encompasses events "'from ancient times things have not been done'" (Isa 46:10). Only God can accomplish this. He provides the perspective saying, "'I am God, and there is no other; I am God and there is no one like Me'" (:9). "No prophecy was ever made by an act of human will" (2Pe 1:21). God declares, "'I have planned it'" (Isa 46:11) and it is "My purpose'" (:10). Isaiah prophecies fulfillment. It "'will be established'" (:10), "'I will accomplish all'" (:10), "'I will bring it to pass'" (:11), and "'surely I will do it'" (:11).
God instituted the Feasts of the Lord to atone for the sins of Israel. The animal sacrifices have their fulfillment in Christ's death on the Cross. This plan was introduced long ago and was practiced over a long period showing that God's intentions are not just ideas that "seemed good for the moment." The chance that the hundreds of prophecies altogether would come to pass as predicted accidentally is statistically small. Of course, if the prophecy was general lacking detail then an interpreter would have more latitude in explaining it in his own way. In fact, various religions often interpret a scripture so it reflects their own belief thereby invalidating your meaning in their eyes. It would be advantageous if the content of the prophecy didn't contain its own obvious explanation because that would eliminate the mystery. Also, there is a span of time between the prediction and its fulfullment such that the person speaking it does not have the opportunity to fulfill it by himself. In fact, it could be argued by some that since Christ knew the scriptures he could have manipulated events to cause them to happen that way. But there would have been too many imponderables and ramifications to make this possible. You must give God the credit for having the understanding and insight for knowing what would happen given certain circumstances. You must also recognize that God has the power to determine outcomes without having to override peoples' freedom of choice.