While copying from 𝓞 into 𝓟, Oliver Cowdery initially wrote “in the day of temptation” but nearly immediately corrected the singular day to days by inserting the plural s inline (there is no change in the level of ink flow). The plural days is probably correct since just previously Oliver had initially written “in the days of ” and then corrected it to “in the provocation” (by crossing out days of and supralinearly inserting provocation, spelled by Oliver as provication).
Jacob’s language here parallels two biblical passages that read as follows in the King James translation (the Old Testament passage is the source for the New Testament one):
Both these biblical passages have the singular day in “in the day of temptation”. This reading suggests the possibility that the original text in Jacob 1:7 could have also read “in the day of temptation”. However, the fact that the singular day was virtually immediately corrected to days and that previously Oliver incorrectly wrote days argues that the original manuscript (not extant here) had days. It is still possible, of course, that the original text itself had day and that earlier, when Joseph Smith dictated this passage to Oliver Cowdery, the plural days was accidentally substituted for day. There is considerable evidence that Oliver Cowdery occasionally mixed up day and days in the manuscripts (for a list of seven instances, see under 2 Nephi 25:8).
On the other hand, there is at least one case where the Book of Mormon text apparently disagrees with a corresponding biblical passage in the number for day:
In this case, the earliest Book of Mormon text has the singular day and the biblical text has the plural days (the reverse of the situation here in Jacob 1:7). However, this example may involve a scribal mishearing of “days shall” as “day shall”. For discussion, see 2 Nephi 23:22.
Since the plural days will work in Jacob 1:7 and it appears to be supported by the earliest textual source (namely, the printer’s manuscript), the critical text will accept “in the days of temptation”, in contrast to the biblical phraseology (“in the day of temptation”).
Summary: Accept the plural days in Jacob 1:7 (“in the days of temptation”), the corrected reading in 𝓟 and the probable reading in 𝓞; even so, it is possible that the original text read “in the day of temptation”, just like in Psalm 95:8 and Hebrews 3:8.