This is a long read, but there are important elements of feudalism in how this all happened. And Eastern feudalism is a prerequisite for the Covenant.
David had 20 sons; 11 were considered in line for the throne. Amnon was the eldest, killed by his half-brother Absalom for raping Tamar, and then rejecting her. Kileab was second, and is hardly mentioned, indicting he probably died young. Third was Absalom, who died in battle against his own father. Fourth, and surviving eldest son, was Adonijah.
David was suffering from hypothermia, a common ailment in elderly people — he could no longer generate enough heat on his own. The customs of the day called for a young woman who was technically part of the harem, but would remain eligible for marriage later, since her only service to the King was sharing her body heat.
Adonijah declared himself regent, meaning he ruled on behalf of his father. His father wasn’t dead, but was inactive, and David had never formally announced the nomination of Solomon as his successor. So Adonijah played it up with aping Absalom’s habit of having a boisterous company of soldiers running down the road to clear the way for his chariots. He enlisted key figures in the royal court to support his pretense of being regent.
However, other more significant figures didn’t join this party. Adonijah decided to host a feast to celebrate the announcement of taking up the regency. He chose a collection of buildings standing on or near the En-rogel Spring as the site for this feast. It stands down just below where the Kidron and Hinnom Valleys run together south of David’s capital. The image you see here was taken in 1857 before the area became so built up. It shows a view up the Kidron Valley from En-rogel (buildings in the lower right foreground) and the scale of the ridgeline on which the City of David stood (left), against the Mount of Olives on the other side. If you can pick out the road wandering down from the Temple Mount on the right shoulder, just about where it disappears at its bottom is roughly where the Gihon Spring was.
Adonijah excluded the court prophet, Nathan, along with Benaiah, David’s chief of security, and Solomon, his youngest royal brother. From what we can tell, Solomon was not yet regarded as an adult (roughly 30 years old), so that would have been his excuse. This is probably why David had delayed the official vestment. However, the whole picture is one of Adonijah knowing his position wasn’t all that secure.
Nathan as Court Prophet was more aware of the political maneuvering than anyone else. His exclusion from the party didn’t keep him from overhearing all the planning and plotting. On top of that, he had the prophetic gift of insight into the meanings of these things. He warned Solomon’s mother, Bathsheba, of the threat this activity meant to her. Adonijah was no better than Abasalom when it came to plotting to murder his rivals, along with anyone else who got in his way, once David actually died.
Nathan also knew how to get David’s attention. He advised Bathsheba to have an audience with the King about this matter. Nathan would then come in the door behind her and also discuss it with David in his own official capacity. David got the point; freezing or not, he had to rise far enough physically to conduct some official business. So he called Bathsheba back in and confirmed his promise to her about Solomon. He was going to publicly vest Solomon as regent that very day.
David ordered the proper rituals so that this was done right. It included his own marked mule and all the pomp. The ritual would take place at the Gihon Spring, the actual water source of the city. At a minimum, this is where public proclamations took place, but it was also just below the terrace where the Ark of Covenant stood. Thus, once the solemn ritual took place, Solomon would ride into the eastern gate of the citadel with music and shouting. Every element was symbolic of royalty coming to claim his throne. Then the ritual would end with Solomon literally taking possession on David’s throne.
Try to imagine the scene with Adonijah’s party, hearing the shofar wailing down the valley at him. Then he looks up to see this coronation with all the most ancient rituals, and the population of the city celebrating with priests in attendance, along with the King’s own official bodyguard (Cherethites and Pelethites) in their unique uniforms.
Add to that the announcement from the son of the Priest, and the festivities at En-rogel simply died. Everyone quietly parted from there in different directions. By this time there was a small stone altar in front of the Ark of Covenant, and Adonijah made his way there to grasp two of the horns poking up at the corners. This was to symbolize his capitulation; no one was going to execute him at the altar for his act of rebellion at the risk of God’s wrath. He announced his petition to spare his life.
When Solomon learned of this, he agreed on certain terms. If Adonijah would walk by the Covenant and humbly embrace his lot at the decision of his own father, then he was safe. If he continued to show arrogance, he would not live long. So on those terms, Adonijah was escorted from the altar to Solomon’s throne, where he fell on his face according to proper protocol. Solomon dismissed him in peace; “Go home.”
