@Elísabet *****************
, You are staying for 32 days, not 31 days. Check James's comment.
As people mentioned you CAN get an SETV before you travel, but it's not a "must". But having SETV in advance could:
- save you hassle with your airline on check-in as many air-carrier ask for an onward flight ticket within 30 days when you don't have a proper visa
- save you a bit of money as the SETV application fee at the embassy is cheaper than the extension fee at the local immigration office
- prevent you from spending half to one day (depending on the province where you stay) at the immigration office to apply for an extension during your 32-day holidays