Art. 30.6: if the day is a working day where you live and not at the seat of the authority, or the other way round, “it shall be treated as non-working in every case”. One town’s local holiday extends the deadline for both sides.
Art. 37.2 of the Workers’ Statute guarantees only four holidays across the whole of Spain: New Year, 1 May, 12 October and Christmas. The rest is set by each year’s calendar and a region may substitute them.
This result uses the national calendar and no regional or local one. Add yours below if the interval touches any.