Each of these actions conveys how emotionally distraught Nehemiah was on account of what he had just heard (Job 2:12–13). The sitting suggests physical weakness, the weeping the physical expression of grief in the form of wet eyes and convulsions of the lungs, the mourning the broken heart that knows no comfort. The intensity of his distress receives further expression in its duration, that is, for days.
The precise reason for his distress will become evident from the prayer he will pray.
4 As soon as I heard these words I sat down and wept and mourned for days, and I continued fasting and praying before the God of heaven.