This is the 101st most frequent Ukrainian word.
дня
The Ukrainian word "дня" means "of the day" or "day's" (genitive singular form of "день").
The word 'дн'а' is the genitive singular form of 'день', relating to 'a day', used here to specify a particular day characterized as wonderful.
Це був чудовий день.
It was a wonderful day.
Here, 'дня' is again the genitive singular, used after 'під час' ('during') to indicate the time frame of the activities described.
Під час цього дня ми відвідали багато музеїв.
During this day, we visited many museums.
In this sentence, 'дня' is the genitive singular of 'день', used within the phrase 'до кінця' ('by the end') to denote a point in time.