der Sonntagabend
Sunday evening
The part of the day at the end of a Sunday, typically after 6 p.m.
Am Sonntagabend schaue ich immer den Tatort im Fernsehen.
On Sunday evenings, I always watch Tatort on TV.

Sunday evening
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Sunday evening
The part of the day at the end of a Sunday, typically after 6 p.m.
Am Sonntagabend schaue ich immer den Tatort im Fernsehen.
On Sunday evenings, I always watch Tatort on TV.

The word is a simple compound of 'der Sonntag' (Sunday) and 'der Abend' (evening). It literally refers to the evening of Sunday.
Wir verbringen den Sonntagabend oft mit der Familie.
We often spend Sunday evening with the family.
To express that something takes place at this time, the preposition 'am' is used. The question for it is 'Wann?' (When?).
Am Sonntagabend gehe ich gerne früh ins Bett.
On Sunday evening, I like to go to bed early.
This word follows a very common pattern in German. You can combine any day of the week with a part of the day such as 'Morgen' (morning), 'Mittag' (noon/midday), or 'Abend' (evening), e.g. 'Montagabend' (Monday evening) or 'Freitagmorgen' (Friday morning).
Der Film läuft immer am Dienstagabend.
The film is always on Tuesday evening.
evening
'der Abend' and 'der Sonntagabend' both refer to a time of day at the end of the day. 'der Abend' is general and can be on any day. 'der Sonntagabend' is more specific: it is the evening on a Sunday. Learners may confuse the words if they are only thinking about the time of day. If the day of the week is important, 'Sonntagabend' fits. If only the time of day is meant, 'Abend' is enough.

Am Sonntagabend spielen wir oft Fußball mit den Nachbarn.
On Sunday evenings, we often play soccer with the neighbors.
Am Abend ruft der Kunde noch einmal im Büro an.
In the evening, the customer calls the office one more time.