Upon purchase of a property in Spain, non-resident owners are liable to pay a tax based on the rateable value of the property, once a year.
The tax is payable one year in arrears, so if you were to purchase a property in 2019, the tax would be payable by 31/12/2020.
The first years tax is payable pro rate the portion of the year that you have owned the property, so if you purchase on 1st October 2019, you will be liable for a quarter of the normal tax payable (with the previous owner liable for the remainder).