Here the printer’s manuscript reads “many day repentance”. The 1830 typesetter interpreted many day as a modifier of repentance; thus he inserted a hyphen between many and day. But the 1837 edition corrected the reading many-day to the adverbial phrase many days. Another possibility, a theoretical one, is that the original text here in Helaman 13:2 read many a day; but as explained under Mosiah 18:7, there are quite a few occurrences in the original text of the adverbial “many days” but none of “many a day”. The expression “many a ” is found a few times in the King James Bible: “many a time” occurs three times in Psalms, and “many a curse” is found in Proverbs 28:27. Still, there are no examples of “many a ” in the Book of Mormon, so the original text for Helaman 13:2 probably did not read “many a day”.
Similarly, there is no independent evidence in the Book of Mormon text for adjective phrases like many-day. But there is considerable evidence in the manuscripts that days was sometimes written as day (for a list of examples, see under Helaman 8:16). In fact, in Mosiah 18:7 the printer’s manuscript reads after many day, the same error as here in Helaman 13:2. The 1830 typesetter made the change to “many days” in Mosiah 18:7, while in Helaman 13:2 the 1837 edition made the change. In both instances, the critical text will follow the emendation to the adverbial phrase “many days”.
Summary: The 1837 emendation “many days” in Helaman 13:2 is undoubtedly the reading of the original text and most likely the reading of the original manuscript; the earliest extant source is the printer’s manuscript and it reads many day; there is no evidence that this phrase is acting as an adjective, nor is it an error for “many a day”.