Please look at the question in PORTUGUESE. 

Ola,

 Sera que alguem poderia me ajudar!!!    Obrigado

 Qual das duas formas esta correta?  ...... em cada segunda semana do mês .... ou

                                                          ...... na segunda semana de cada mês.

Correct me if I am wrong, but option 1 appears to assume that there are many "second weeks" in a given month.  Option 2 points to the second week that occurs every month -  there is a second week in January, there is a second week in February, etc, etc...  What is the correct form in Portuguese?  I think it is option 2!

Languages can be amazingly flexible.  You can, of course, say ANYTHING you want, in any way you want it, but, in my humble opinion, the purpose of a Language is to communicate ideas clearly and effectively, keeping in mind your target audience in THEIR language, culture, and  context.  For this question, option 2 does that better than option 1.

Be well.

CR