This is the 13173rd most frequent German word.
anstrengende
Demanding or exhausting.
In this sentence, 'anstrengenden' is in the dative case due to the preposition 'nach', describing the day as exhausting.
Nach einem anstrengenden Tag gehe ich früh ins Bett.
After a tiring day, I go to bed early.
Here, 'anstrengende' modifies the noun 'Wanderung', describing the hike as physically demanding.
Das war eine anstrengende Wanderung.
That was a strenuous hike.
Here, 'anstrengende' describes the meeting, conveying that it was mentally or emotionally taxing.