This is the 146th most frequent Italian word.
ottobre
"October" (the tenth month of the year).
In this sentence, 'ottobre' is used as a noun to refer to the month of October.
Ottobre è il mese in cui le foglie cominciano a cadere.
October is the month when leaves start to fall.
In this case, 'di ottobre' is part of a prepositional phrase connecting 'ottobre' to the noun 'clima' to indicate possession or association.
Mi piace il clima fresco di ottobre.
I like the cool weather of October.
Here, 'a ottobre' functions as a prepositional phrase indicating the time during which the action occurs.