Gloom of night

"Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds."

That's what it says on an inscription on the James Farley Post Office in New York City. But in reality, the post office shuts down during the gloom of night. The delivery trucks are lined up and ready for the next day's mail, when neither rain nor heat will keep our mail carrier from showing up with the mail.