Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
vespĕrus, a, um, adj. [vespera], of or belonging to the evening, evening-: his horae rigandi matutina atque vespera, Plin. 19, 12, 60, § 183 (dub.; al. matutino atque vespera): tempus vesperum, Cael. Aur. Acut. 1, 15, 141; 1, 15, 151.