As a general rule, Spanish VAT returns must be submitted and paid by the 20th day of the month following the reporting period. Exceptionally, the December and fourth quarter VAT return are due by the last day of January following the reporting period. If the due dates falls on a Saturday, Sunday or public holiday, the date is shifted to the next working day.
The 2016 due dates for all taxes in Spain is published (in English) in the tax authorities’ website.
Regarding bank holidays in Spain, as there are several differences per region, we suggest taking into account only those bank holidays that apply in the whole Spanish territory. In any case, the due date calendar published by the authorities every year includes all these dates. You can access here the official national and regional bank holidays in Spain.