The committee for the 1920 LDS edition removed the preposition in here at the beginning of Jacob 6:2, thus equating the two days (“the day that S 1 is the day that S2”, where S1 and S2 stand for finite clauses). This editing rejects the idea of one day being included as part of another day. Yet the word day in this passage stands for a period of time, not a literal 24-hour day, so the second day can actually be considered a part of the first. In fact, the explanatory “yea even the last time”, in reference to the second day, implies as much. In other words, the meaning of the text is:
and in the period of time that he shall set his hand again the second time to recover his people is the last period of time that the servants of the Lord shall go forth
It is possible, of course, that the preposition in was accidentally introduced during the early transmission of the text, perhaps because of the familiarity of the phraseology “in the day that”. This language occurs seven other times in the original Book of Mormon text, as in Omni 1:9: “and he wrote it in the day that he delivered them unto me”. But more significantly, this phraseology would have been familiar to the scribes from the creation story in the King James Bible:
Even so, there is no evidence in the manuscripts (or the printed editions, for that matter) of the preposition in being accidentally introduced into the text unless there already is an in elsewhere in the same sentence (for an example, see the discussion regarding the intrusive in in 2 Nephi 11:5). The critical text will accept the earliest reading in Jacob 6:2 since the in is possible when the word day is figuratively interpreted as a period of time.
Summary: Restore in Jacob 6:2 the original “and in the day that” since the word day can be metaphorically interpreted as referring to a period of time of varying length, thus allowing a day to be within a day.