The original manuscript is not extant here. The 1830 edition has the singular day; the printer’s manuscript has the plural days. The more frequent tendency in the text has been to drop the plural s from days. For a list of at least six examples where Oliver Cowdery replaced days with day, see under Helaman 8:16. For one more possible example, see the discussion under Alma 50:23. There are a few examples in the history of the text where a singular day has been replaced with the plural days; but all of the clear examples are found in the printed editions, and only one of them (marked below with an asterisk) has persisted beyond a single edition:
Here in Helaman 15:16, the singular phraseology, “in the day of my wisdom”, is clearly the better reading. The Lord is referring to a day of his own choosing, in his own wisdom, when the Lamanites will be restored to the truth and “return again unto me”. Moreover, the text otherwise uses the singular day when referring to the time of the restoration of the house of Israel (including the restoration of the Lamanites in the last days). The singular day is found in some 23 other passages like Helaman 15:16, as in these examples:
More generally, the Book of Mormon consistently uses the singular day to refer to any major event in the last days: the day of the restoration of the gospel, the day of the second coming, the day of resurrection, the day of judgment, and so on. Usage therefore supports the use of “in the day of my wisdom” in Helaman 15:16, the reading of the 1830 edition (and both the current LDS and RLDS texts).
Summary: Accept in Helaman 15:16 the singular reading “in the day of my wisdom” (the 1830 reading) since in 23 other references to the restoration of Israel (including the restoration of the Lamanites) the Book of Mormon consistently uses the singular day.