der Donnerstagabend
Thursday evening
The evening of the day Thursday.
Wir treffen uns immer am Donnerstagabend zum Stammtisch.
We always meet on Thursday evening for our regulars' table.

Thursday evening
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Thursday evening
The evening of the day Thursday.
Wir treffen uns immer am Donnerstagabend zum Stammtisch.
We always meet on Thursday evening for our regulars' table.

This word is made up of 'Donnerstag' (Thursday) and 'Abend' (evening). So it literally refers to the evening of Thursday.
Was machst du am Donnerstagabend?
What are you doing on Thursday evening?
To express a specific point in time, you use the preposition 'am'. This combination usually appears at the beginning or end of the sentence.
Am Donnerstagabend gehe ich ins Kino.
On Thursday evening, I'm going to the cinema.
When something happens regularly on this weekday, you use the adverbial form 'donnerstagabends' (on Thursday evenings). Note that it is then written in lowercase and has an 's' at the end.
Ich spiele donnerstagabends immer Volleyball.
I always play volleyball on Thursday evenings.
Thursday
'der Donnerstag' and 'der Donnerstagabend' refer to the same day of the week, but not to the same time period. 'Der Donnerstag' means the whole day between Wednesday and Friday. 'Der Donnerstagabend' means only the evening of that day. The words can be confused when planning an appointment. If the exact time of day is not important, or if the whole day is meant, you say 'Donnerstag'. If it is clearly about the evening, 'Donnerstagabend' is more precise.

Für den Donnerstagabend haben wir Karten fürs Kino gekauft.
We bought tickets for the cinema for Thursday evening.
Am Donnerstag habe ich frei und kann zum Arzt gehen.
I have Thursday off and can go to the doctor.