- The angel Moroni entrusted the plates into the care of Joseph Smith on 22 September 1827.
- Jews across the world were celebrating the Feast of Trumpets on this same day.
- Jewish Tradition also celebrates this day as a time of remembrance (specifically that God remembers His promises) and begins a period of repentance in anticipation of the day of Atonement, which follows after the Festival of Trumpets.
- This festival presents a very clear symbolic metaphor for the gathering of the House of Israel prior to the second coming of Jesus Christ.
According to this January 2000 Ensign article:
Because this festival was set for the first day of the seventh month of the Judaic calendar (usually falling in our September), the day was timed for the gathering of the vital oil and wine. Thus this festival initiated the fall season and the great and final time of “ingathering.”
Many Jewish scholars have taught that the final spiritual gathering of Israel would begin with the Feast of Trumpets. (source)
We've noted before that the meaning of the name "Joseph" is "God shall add to" or "God will proceed to." This makes the name symbolic of God remembering His promise to Israel. Jewish tradition also teaches that Sarah, Hannah, and Rachel all conceived on this specific day. Joseph (Rachel's son) was freed from prison on this day.
One of the consistent teachings in the Book of Mormon is that God will remember His promise to Israel in the latter days.
11 Wherefore, the Lord God will proceed to [yasap] make bare his arm in the eyes of all the nations, in bringing about his covenants and his gospel unto those who are of the house of Israel. (1 Nephi 22)This verse from Isaiah is read on the Festival of Trumpets:
13 And it shall come to pass in that day, that the great trumpet shall be blown, and they shall come which were ready to perish in the land of Assyria, and the outcasts in the land of Egypt, and shall worship the Lord in the holy mount at Jerusalem. (Isaiah 27)Considering the the Book of Mormon is the long-promised instrument of the gathering, this symbolism is very apt. The harvest is the gathering. The trumpet is the Book of Mormon and the gospel message calling the world to repentance. That repentance is possible through the restoration of the Everlasting Covenant.
The article continues:
As modern prophets have said, the Book of Mormon is the major instrument the Lord prepared to initiate His final harvest. Therefore, it is significant that the golden plates were received on 22 September 1827, coinciding with the beginning of Israel’s fall garnering and symbolizing the onset of its final harvest of souls...
Was the coming forth of the Book of Mormon on the Feast of Trumpets coincidental? Latter-day Saints who know about these events do not think so. Scriptural and prophetic truth is often manifest through fulfillment. The golden plates were delivered to the young Prophet Joseph Smith early in the morning of 22 September 1827. The Feast of Trumpets, with prayers pleading for God’s remembrance of his still-exiled people, had begun at sundown the previous evening. The services continued that morning, with a worldwide sounding of the ram’s horn. Unbeknown to Judah, all that those horns represented was now to be fulfilled. For on that day, God remembered His people and set in motion His plan to regather them. On that day, God’s final harvest began. On that day, new revelation was granted which would bring a return to renewed covenants. From that day onward, Israel would be called to repentance in preparation for Christ’s return and reign. The Book of Mormon exists to serve these ends. Today, Moroni’s image trumpets from temple spires around the world a final call to awaken, repent, and prepare.