If you are a resident in Spain for tax purposes — usually someone who lives in the country for at least 183 days a year — and earn more than €22,000 a year, Monday is a key date in the Spanish tax year.
"You need to lodge your tax return (declaración de renta) by Monday," Luis González, a spokesperson for Spain's tax office, told The Local.
"This can be done either by internet, or in person at one of our offices."
González said that offices would be very busy today, as many people left handing in their tax returns "to the last moment".
But he said it might be possible to "find a hole" in the schedule.
The tax office spokesperson said that if people were planning to visit an office, he would "highly recommend" that they call and make an appointment (cita previa).
The number for this service is 901 010 210. Appointments can also be made online.
He also warned that the process might involve travelling to an office that was not necessarily the closest one.
González said failure to hand in a tax return by Monday could mean fines.
"If you owe money to the Tax Office, you will be liable to a 5 percent charge within the first three months of being late," he explained.
But he was at pains to point out that people would not have to pay an extra charges if the Tax Office owed them money.
Tax residents in Spain who earn less than €22,000 should lodge a tax return if they have believe they have deductions owing to them.