How to translate dates?
Most of Drupal contributed modules have their translation under Drupal Translations

If you visit the site and search for Date, you will notice list of countries and languages.

I notice Portuguese translations for Date module.


Portuguese, Brazil
Portuguese, International
Portuguese, Portugal


Download the correct language translation file by selecting the version of Date module your using.

Go to http://[your_site]/admin/config/regional/translate/import . To import the downloaded file.

For more information about Translation and how to build multi language site please refer to my answer in https://drupal.stackexchange.com/a/184623/24038