This is the 14104th most frequent Russian word.
октябрь
October.
In this sentence, "Октябрь" serves as the subject of the sentence, and it is used nominally to denote the specific month in the calendar.
Октябрь — это середина осени.
October is the middle of autumn.
In this sentence, "октябрь" is the object of the preposition "на", which indicates the time period (October) during which the action (planning a vacation) will take place.
Мы планируем отпуск на октябрь.
We are planning a vacation for October.
Here, "октябре" is used locatively with the preposition "в" and the prepositional case, indicating the time frame within which the described condition (getting colder) occurs.