Here the 1837 edition changed the singular day (“from his day”) to the plural days (“from his days”). The following plural days in “even to the days of Abraham” may have led to this 1837 change, whether intended or not (the change of day to days was not marked by Joseph Smith in 𝓟). There is definitely internal evidence to support the plural days in phrases of the form “the day(s) of X”, where X is a personal name; for discussion of the phrase “since the day(s) of Nephi”, see under Alma 50:23. But here in Helaman 8:16, there is no postmodification by an of-phrase; instead, the word day(s) is premodified by the determiner his. Elsewhere, the text has quite a few examples of his days (17 of them), but there are also three examples of his day; the first two clearly refer to Christ, and it appears that the third one does too:
So one could claim that the text uses only his day when referring to the day of Christ.
However, when we consider the parallel case of my day(s), we find two instances of my day (and for neither of these does the my refer to the Lord):
Elsewhere there are 11 occurrences of my days. So probably what we have is a predominance of his days and my days, but the singular day is still possible for both his day and my day. Similarly, his days and my days dominate in the King James Bible, but there are also a few instances of his day and my day, as in Job 3:1 (“after this opened Job his mouth and cursed his day”) and in John 8:56 (“your father Abraham rejoiced to see my day”), the latter referring once more to Christ. The critical text will restore the singular his day here in Helaman 8:16.
The possibility remains, of course, that his day here in Helaman 8:16 is an error for his days. There are quite a few examples in the manuscripts where the scribes accidentally wrote day instead of the correct days (and sometimes without correcting the error in the manuscript):
Note that the example in Alma 13:18 deals with the phrase “in his days” while the one in 3 Nephi 2:5 deals with “since the days of Mosiah”. Thus the case of “in his day” here in Helaman 8:16 could be an error.
Summary: Restore in Helaman 8:16 the singular day in the phrase “in his day”; the singular is possible, although the text prefers the plural phrase “in his days”.