One wonders if the phrase “in the latter day” is an error for “in the latter days”. This is the only occurrence of the singular day for this phrase in the text. Elsewhere, we get only the plural form “in the latter days” in the original text (five times), including this one where Oliver Cowdery initially wrote “in the latter day” in 𝓟:
𝓞 is extant here and reads days. Thus Oliver could have accidentally written “in the latter day” in 𝓞 for 3 Nephi 16:7.
Usage in the King James Bible also supports the plural phraseology “in the latter days” (with 11 occurrences). There are no instances of “in the latter day” in the King James Bible, but there is one of “at the latter day”:
More generally, we find that the singular phrase “in the latter day” was fairly common in Early Modern English. The Oxford English Dictionary provides this example under definition 3a for the adjective latter (original spelling retained):
The OED also provides support for the phrase “at the latter day”, the reading in Job 19:25 (original spelling retained):
Literature Online provides many examples of “in the latter day”, as in these two examples from Early Modern English that refer to the resurrection and the judgment that will occur in the last days (original spelling again retained):
There are also examples of “in the latter day” in the 1800s, although they occur less frequently than in Early Modern English. The critical text will retain the one instance of “in the latter day” in the Book of Mormon text (here in 3 Nephi 16:7). Of course, the possibility remains that this is an error for “in the latter days”.
Summary: Maintain in 3 Nephi 16:7 the singular day in the phrase “in the latter day”, even though elsewhere the Book of Mormon text has instances of only “in the latter days”; usage from Early Modern English up into the 1800s supports the occurrence of “in the latter day”.