This is the 342nd most frequent Polish word.
dniu
The Polish word "dniu" means "day" in the locative or dative case.
Here, 'dniu' is the locative case of 'dzień', indicating time when something happened.
W tamtym dniu byłem szczęśliwy.
On that day, I was happy.
Here, 'dniu' is the locative case of 'dzień', referring to time in a general habitual context.
O każdym dniu myślę z nadzieją.
I think about each day with hope.
Here, 'dniu' again indicates the specific time when something took place, used with the preposition 'w' for a particular case.