Mormon 8
I can imagine that after the battle of Cumorah Mormon and moroni stayed close enough together that they could keep track of each other and Moroni thus knew when and how his father died. In fact, it may well have been only at the command of Mormon that they did not travel together which command would have been given because Mormon knew that he would be sought after more than his son, and he wanted to preserve the records. The other option is that they were generally together and happened to be separate at the time that Mormon was found.
I wonder which people Moroni had specifically in mind as he wrote that he had “seen your day?” Was it the people of the 1830’s or the people of 2000 – or just both generally? I’m sure his words apply to both.