Redaction analysis: Nephi gives us a tantalizing hint as to the nature of his writing, but it isn't clear enough that much can be made of it. There are at least two possibilities attendant to "Jacob spake many more things to my people at that time."
The first is that Nephi recorded the only two times that Jacob spoke, but didn't record everything that Jacob said in those two sermons. This does not appear the most likely case, however, because Jacob's addresses as recorded appear to have a reasonably strong organization. It would be hard to see where things were left out such that Nephi would claim them as "many more things."
This leaves the more probable answer that in addition to the two sermons Nephi cites for that occasion, Jacob spoke to the congregation at least one more time. While not certain, this is the more likely of the two possibilities. Given the split in the sermons from day one to day two, and the length of the second days sermon (not to mention its completeness and finality) it is also quite probable that the other things Jacob spoke came on the next day. Thus we are looking at a multiple day festival with a minimum of three days.
If Jacob does have another sermon on this day, why isn't it recorded along with the sermon that covered the first two days? The best probably reason is that this is Nephi's record, not Jacob's, and the sermon given on the first two days was directly at Nephi's suggestion (2 Nephi 6:4). This text is here not because of Jacob's interest in it, but because of Nephi's interest in the sermon. This was given under Nephi's direction, and for his purposes, and therefore warranted a place in Nephi's personal writings.