This is the 125th most frequent Polish word.
ostatnich
"Last" or "recent" (genitive plural form).
The word 'ostatnich' is used here as a genitive plural adjective, modifying 'dni' (days), referring to the recent days.
W ciągu ostatnich dni, pogoda była bardzo zmienna.
In the last days, the weather has been very variable.
In this sentence, 'ostatnich' is a genitive plural adjective describing 'wakacji' (vacation), denoting the specific past vacation.
Podczas ostatnich wakacji odwiedziliśmy wiele ciekawych miejsc.
During the last vacation, we visited many interesting places.
Here, 'ostatnich' functions as a genitive plural adjective linked to 'czasach' (times), indicating a recent period.