der Montagabend
Monday evening
The evening of the weekday Monday. Refers to the period from late afternoon until night.
Am Montagabend schaue ich immer meine Lieblingsserie.
On Monday evenings, I always watch my favorite series.

Monday evening
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Monday evening
The evening of the weekday Monday. Refers to the period from late afternoon until night.
Am Montagabend schaue ich immer meine Lieblingsserie.
On Monday evenings, I always watch my favorite series.

The word is clearly made up of 'Montag' (Monday) and 'Abend' (evening). This pattern is very common and also works with all other weekdays and times of day, such as 'Dienstagvormittag' (Tuesday morning) or 'Sonntagnachmittag' (Sunday afternoon).
Der Montagabend ist oft der ruhigste Abend der Woche.
Monday evening is often the quietest evening of the week.
To express a specific point in time, 'Montagabend' (Monday evening) is almost always used with the preposition 'am'. The question for this is 'Wann?' (When?).
Am Montagabend treffen wir uns zum Essen.
On Monday evening, we're meeting for dinner.
For regular, repeated actions, there is the lowercase form 'montagabends'. The 's' at the end signals repetition and means 'every Monday evening.'
Montagabends gehe ich immer zum Sport.
On Monday evenings, I always go to the gym.
Monday morning
'der Montagmorgen' and 'der Montagabend' both refer to part of the same day of the week, Monday. The difference is the time of day: 'Montagmorgen' means the early part of the day, especially the time before and at the beginning of the morning. 'Montagabend' means the late part of the day, when work, school, or other appointments are often already over. So the contrast is not the day of the week, but morning versus evening.

Der Montagabend war ruhig, weil das Training ausgefallen ist.
Monday evening was quiet because training was canceled.
Am Montagmorgen ist der Zug oft besonders voll.
On Monday mornings, the train is often especially crowded.