The deadline of the end of December has obviously been extended as there is apparently still time for foreigners who have been on the padrón (census) for more than 5 years, and have not made any changes in the past two years, to renew their registration.
Failure to renew will lead to removal from the padrón. To check that you are still registered on the padrón, just pop along to the main reception on the ground floor of the Town Hall from Monday to Friday between 08:00 and 13:30 with your NIE certificate and your passport (copies are acceptable). You can also go to the Foreigners Department on Mondays or Thursday mornings.