Lyle Fletcher (personal communication, 19 May 2008) wonders if the plural days here in Ether 13:4 is an error for day. In the discussion under Alma 50:23, I note that the Book of Mormon text definitely prefers the plural expression “the days of X”, where X is a personal name. As noted in that discussion, Christ could be considered a title rather than a personal name. Nonetheless, the plural days is expected in this expression; the only example in the earliest text with day is “the day of Nephi” in Alma 50:23, but as explained under that passage the critical text will accept the 1830 emendation to “the days of Nephi”. Here in Ether 13:4, the critical text will maintain the plural days in “the days of Christ”, the earliest extant reading (the reading in 𝓟 since 𝓞 is not extant here).
Summary: Maintain in Ether 13:4 the plural days in “the days of Christ”, the reading of the earliest textual sources; usage elsewhere in the text supports the plural days in the phrase “the days of X”, where X is a name.