The 1830 typesetter accidentally replaced the preposition at with in. Yet both phrases are found elsewhere in the Book of Mormon text: “at that day” occurs 23 times, and “in that day” occurs at least 40 times (see the discussion regarding “in that day” under Helaman 14:20).
The 1830 error here in Alma 5:17 is probably the result of “in that day” occurring in the preceding verse:
In specific support of “at that day”, there are nearby examples of that phrase a few verses later:
Thus there is no reason for changing “at that day” to “in that day” in Alma 5:17; the critical text will restore the original preposition, at.
Summary: Restore “at that day” in Alma 5:17, the reading of the earliest textual source (the printer’s manuscript).