I am doing volunteer work every month at an event organized by a big company (not mine). Now they invite me to a 感謝の夕べ:
日頃お手伝いいただいているボランティアスタッフへの感謝の気持ちと共に
より懇親を深めるため、XYZにて「感謝の夕べ」を開催いたします！
Searching the web reveals that 感謝の夕べ is a popular concept, but I could not find a clear definition.
What does the word convey, opposed to 懇親会 for instance?
懇親会 attendees usually have to pay, is it also the case for 感謝の夕べ?
～の夕べ is nothing more than a way to name an event held in the evening. In English, it's probably just "～ Night". For example, when we hear クラシック音楽の夕べ, we can guess it's an event where attendees will enjoy classical music. I feel it tends to be used as the name of some sophisticated/classy event rather than a noisy party, but this may not apply in your case.
I suppose you probably don't have to pay because you are a guest who will receive their 感謝, but that is by no means a grammatical rule.
