Isaiah 10:3 (King James Bible) and what will ye do in the day of visitation and in the desolation which shall come from far
Once more the 1953 RLDS edition has replaced a singular with a plural. The change here may be intentional since that edition has a footnote referring the reader to Hosea 9:7, which reads in the plural: “the days of visitation are come / the days of recompense are come”. For a list of other places where the 1953 RLDS edition replaced a singular with a plural, see 2 Nephi 7:3. Some of these other changes seem to be intentional, others accidental, which suggests the possibility that the plural days may have accidentally entered the copy-text for the 1953 edition and that the footnotes were subsequently prepared from that copy-text. It is also possible that the reference to Hosea 9:7 was discovered first and that it motivated the change to the plural days in the RLDS text. In either case, the Isaiah text reads in the singular and the Book of Mormon text is quoting Isaiah, so this 1953 change to the plural days is clearly unwarranted from a textual point of view.
Summary: Maintain in 2 Nephi 20:3 the singular day, the reading of the earliest textual sources as well as the King James Bible; the 1953 RLDS change to the plural may be based on Hosea 9:7.