Italian Grammar Question- "in" vs "a"
When trying to make sentences, I often get confused as to which I'm supposed to use to mean "in".

"Scriviamo a loro ad novembre." seems straightforward enough, both using "a" (or, well, "ad" before a vowel but I get that.)

Yet, "Non in ottobre, ma in novembre."

Can someone please explain why you would use one over the other?

Question

Italian Grammar Question- "in" vs "a"
When trying to make sentences, I often get confused as to which I'm supposed to use to mean "in".

"Scriviamo a loro ad novembre." seems straightforward enough, both using "a" (or, well, "ad" before a vowel but I get that.)

Yet, "Non in ottobre, ma in novembre."

Can someone please explain why you would use one over the other?

Answer

I believe that in everyday italian you can use both prepositions, although A sounds better. A novmbre, a gennaio, ad ottobre (because there is vowel) etc. But when you want to precise like in the sentence
La guerra è scoppiata nel settembre del 1939.
Nel novembre del 2000 etc, you must use the preposition IN+article, the forms nel, nell'...

Answer

The preposition “A” means “to” (movement) or “in” if it indicates location.

Examples: Tu dai la penna A Simona, (You give the pen to Simona)
Sono A casa. (I’m at home)
Abito A Roma, ma ora sono A Venezia (I live in Rome, but now I’m in Venice)

You have to use the preposition “IN”, with names of continents, States, Nations, region and with words that end in “-eria”.

Examples: IN Inghilterra bevi tè tutti i giorni. (In England you drink tea every day)
Bologna si trova IN Emilia-Romagna (Bologna is in Emilia-Romagna)
Di solito compro i libri IN quella libreria (I usually buy books in that bookshop)

So "Non in ottobre, ma in novembre." is wrong. '' 
''Non a ottobre ma a novembre.'' is better.

I hope you understand,my English isn't perfect.