der Mittwochabend
Wednesday evening
The evening of Wednesday. Refers to the period from late afternoon until night on Wednesday.
Wir gehen am Mittwochabend immer zusammen essen.
We always go out to eat together on Wednesday evening.

Wednesday evening
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Wednesday evening
The evening of Wednesday. Refers to the period from late afternoon until night on Wednesday.
Wir gehen am Mittwochabend immer zusammen essen.
We always go out to eat together on Wednesday evening.

The word is made up of 'Mittwoch' (Wednesday) and 'Abend' (evening). As with all such time expressions, the last word ('der Abend') determines the article of the whole word.
Der Mittwochabend ist für mich der Beginn des kleinen Wochenendes.
Wednesday evening is the start of the little weekend for me.
To talk about a specific, one-time Wednesday evening, you use the preposition 'am'. This stands for the combination 'an dem'.
Was machst du nächsten Mittwochabend?
What are you doing next Wednesday evening?
To express that something happens every Wednesday evening, you use the lowercase adverb form 'mittwochabends' (on Wednesday evenings). This word does not need a preposition.
Mittwochabends gehe ich immer zum Sport.
On Wednesday evenings I always go to sports practice.
evening
'der Abend' and 'der Mittwochabend' both refer to a time at the end of the day. 'der Abend' is general and can mean any evening. 'der Mittwochabend' is more specific: it is only the evening on Wednesday. You can confuse the words when talking about plans or appointments. If the day of the week is important, use 'Mittwochabend'. If only the time of day is important, 'Abend' is enough.

Zu Hause ist der Mittwochabend für den Onlinekurs reserviert.
At home, Wednesday evening is reserved for the online course.
Am Abend wird es in der Nachbarschaft meistens ruhig.
In the evening, the neighborhood usually becomes quiet.