The 1953 RLDS edition removed, probably accidentally, the relative pronoun that in Helaman 15:2. The original manuscript is not extant here, but spacing between extant fragments supports the original occurrence of that. And here the 1830 edition and the printer’s manuscript independently support this reading.
When “in the day” is postmodified by a relative clause, the relative pronoun is normally that (seven times in the original text, including the example here in Helaman 15:2; see the list under Helaman 14:20). In another case, the relative pronoun is the adverbial when: “and I rejoice in the day when my mortal shall put on immortality” (Enos 1:27). But there is one place in the earliest text where the relative clause lacks the relative pronoun: “our swords are taken from us in the day we have sought them for battle” (Helaman 13:34). For each case of “in the day ”, the critical text will maintain or omit the relative pronoun in accord with the earliest extant text, thus “in the day that they shall give suck” here in Helaman 15:2.
Summary: In accord with the earliest textual sources, maintain the relative pronoun that in Helaman 15:2: “in the day that they shall give suck”.