1 Samuel 10:19
And ye have this day rejected your God, who himself saved you out of all your adversities and your tribulations; and ye have said unto him, Nay, but set a king over us. Now therefore present yourselves before the LORD by your tribes, and by your thousands.
On that day Israel rejected God as its ruler by demanding a human king over themselves. That rejection came although God himself had saved them out of all their adversities and tribulations. The injustice of Samuel's sons only furnished a pretext for the demand, while the elders' underlying motive was discontent and pride. The people preferred the customs and display of the surrounding nations, setting their chosen way against God's rule. Their request was a deliberate effort to set Samuel aside. Asking for a king in this way was a serious sin, not a light matter.
The verse places this at the public gathering at Mizpeh, where Samuel gathered Israel to present themselves before the LORD by tribes and by thousands and families for the selection of the king. The selection was made after earnest prayer for divine guidance by lot, identifying tribe and family and household in turn until Saul was selected, making God's choice public. Saul was absent from the assembly because he felt the weight of the responsibility about to fall on him, so he had to be brought back to the assembly.