This is the 11415th most frequent Russian word.
дне
The word "дне" is the prepositional or dative case singular of "день," meaning "day" in English.
'Дне' in this sentence is the prepositional case of 'день', indicating 'on that specific day'.
В тот дне я гулял по парку.
On that day, I walked in the park.
Here, 'дне' similarly uses the prepositional case to specify a particular context within a day.
В свободном дне я читаю книги.
In my free day, I read books.
'Дне' again conveys the prepositional case, describing the state or experiences during that day.