This is the 5170th most frequent Russian word.
неделей
"Неделей" means "week" in the instrumental or prepositional case in Russian.
'неделей' is used in the instrumental case to indicate a specific point in the time frame referring to 'next week'.
Мы встретимся на следующей неделе.
We will meet next week.
'неделей' is used in the instrumental case to refer to 'last week' as a time period.
Прошлой неделей он был занят.
He was busy last week.
'неделей' instrumental case denotes the duration within which the action was performed.