"The period goes from October to December."
Translation:Il periodo va da ottobre a dicembre.
We would never say this translation in English! More common would be 'The period is from October to December' or 'The period runs from October to December' Apart from the fact that in the Uk we do not very commonly use the word 'period' to denote a certain amount of time anyway, unlike American English. Not sure if this is of interest to anyone but maybe useful if British English is not your first language?
"Dall'ottobre" would be a compound of "da la ottobre" (from October), but it uses the feminine definitive article. "Ottobre" is masculine. The masculine would be "dal ottobre," but Duolingo doesn't seem to use the definitive article at all with months. It offers the translation "da ottobre" as the correct one for this sentence.
Dall' Is A Compound Of Da L', And L' Can Be Short For Either La, Or Lo, And Lo Would Be The Correct Article In This Situation, As 'Ottobre' Starts With A Vowel, However It's Not Used Because You Say "From October", Not "From The October". You Might Say "From The Month Of October", But Usually Just "From October" Works.