This is the 53rd most frequent Russian word.
день
The Russian word "день" means "day" in English.
In this sentence, 'день' refers to the time period of one day, particularly emphasizing its quality.
Сегодня был очень солнечный день.
Today was a very sunny day.
Here, 'день' is used to indicate the entire daytime period devoted to an activity.
Вчера я потратил весь день на работу.
Yesterday, I spent the whole day working.
In this example, 'день' generalizes a routine or repetition of daily occurrences.