Long ago, there was a verb belate, which meant "to make late." From the beginning, belate tended to mostly turn up in the form of its past participle, belated. Eventually, belate itself fell out of use, leaving behind belated as an adjective that preserved the original notion of delay. As you may have guessed, belate and its descendant belated derive from the adjective late; belate was formed by simply combining the prefix be- ("to cause to be") with late. Belated was also once used in the sense "overtaken by night," as in "belated travelers seeking lodging for the night." This sense was in fact the first meaning of the adjective, but it has since fallen into disuse.