In modern English, speakers expect “the light of day”; thus the 1874 RLDS edition omitted the definite article the before day, probably unintentionally. The 1908 RLDS edition restored the correct the in “the light of the day”. Elsewhere, the original Book of Mormon text consistently has “of the day”, never “of day”:
(I exclude from this list the one example where “of the day” is postmodified; in such a situation the definite article the is required: “until the end of the day of probation”, in 2 Nephi 33:9.) The example in Alma 47:14 shows that on two different occasions the the was deleted from the phrase “of the day”: (1) initially by Oliver Cowdery as he was copying from 𝓞 into 𝓟, and (2) by the typesetter for the 1879 LDS edition.
Summary: Maintain the use of the definite article in the prepositional phrase “of the day” in Jacob 6:5.