That curse the day — i.e., Let those who proclaim days unlucky or accursed curse that day as pre-eminently so; or let them recollect that day as a standard or sample of cursing. “Let it be as cursed as Job’s birth day.”
These people are further described as being ready to arouse leviathan (Authorised Version, “raise up their mourning”), or the crocodile — persons as mad and desperate as that. Let the most hopeless and reckless of mankind select that day as the one which they would choose to curse. This seems to be Job’s meaning.