"The next month is October."
Translation:अगला महीना अक्टूबर है ।
अगले is used in two cases.
The first is if it is followed by a plural noun. For instance, अगले महीने मेरे लिए महत्वपूर्ण हैं (The next months are important for me).
The second usage is if the noun is in the 'oblique case'. This means that the noun is followed by a postposition. Eg. अगले महीने में बारिश आने वाली है ('In the next month, there is going to be rain' which has the postposition में). Sometimes the postposition may be implicit like in मैं अगले महीने आने वाली हूँ (I am going to come next month) which has the postposition को that is implicit.