Vesperi & vespere
The Duolingo course teaches us the form vesperi, "in the evening", apparently a locative of vesper, "evening". Does anyone know the difference in meaning with the ablative vespere, which can also mean "in the evening"?
Acc. to the Lewis and Short dictionary:
vesper, ĕris and ĕri (in class. prose mostly acc. vesperum, and abl. vespere, or adverb. vesperi; the plur. not used), m. . . .
You can see examples of the use of both forms in the article linked to.
And, from the Oxford Latin Dictionary, just to be really nerdy about it: