"In the evening the comrades are drunk."
Translation:Vesperi contubernales ebrii sunt.
No, vesperi (dative) is already "in the evening", like "noctu/nocte" (ablative) is "in the night" as an adverb.
Ad usually (at least for us, beginners) means a physical move towards a direction.
Ad vesperum, would be "until/to the evening", because a mani ad vesperum = from the morning to the evening.
The thing that I don't get is why "vesperi" is dative, and "noctu/nocte" is ablative. Why it's not Vespere?
Based on what Wiktionary says: 'the forms vespere and vesperī were both used to mean "in the evening".'
It also states that both can be the ablative and that it was treated as both a second declension and third declension at points. Maybe the use of vesperī as an ablative is a remnant of pre-Classical Latin? I think (if I remember correctly) the -ī was used for the ablative singular (or at least at some point) for the third declension.
The locative case, normally associated with cozy places, can be used with "vesper -eris or eri".
and the locative for "vesper" is "vesperī".
Another alternative is the "ablative of time within which" (which does not require a preposition). For "vesper" the ablative can be various forms, among them, "vespere" and vesperī"
These are deep waters.
All in all, it's probably best just to learn "vespere" and "vesperī" as fixed expressions meaning "in the evening".