Jamieson, Fausset and Brown on Job 3:8

8. them ... curse the day--If "mourning" be the right rendering in the latter clause of this verse, these words refer to the hired mourners of the dead (Jr 9:17). But the Hebrew for "mourning" elsewhere always denotes an animal, whether it be the crocodile or some huge serpent (Is 27:1), such as is meant by "leviathan." Therefore, the expression, "cursers of day," refers to magicians, who were believed to be able by charms to make a day one of evil omen. (So Balaam, Nu 22:5). This accords with Umbreit's view (Job 3:7); or to the Ethiopians and Atlantes, who "used to curse the sun at his rising for burning up them and their country" [Herodotus]. Necromancers claimed power to control or rouse wild beasts at will, as do the Indian serpent-charmers of our day (Psa 58:5). Job does not say they had the power they claimed; but, supposing they had, may they curse the day. Schuttens renders it by supplying words as follows:--Let those that are ready for anything, call it (the day) the raiser up of leviathan, that is, of a host of evils.