Day and month name translation
You can translate Interface strings through Configuration > Regional and language > User interface translation (/admin/config/regional/translate). 

Enter "Submitted by" into the "String contains" search field. It's important to enter correct capitalization of letters or your string won't be found (don't enter "submitted by"). 

Your search will find this string to translate: 

Submitted by @author_name on @date

As you can see you can translate part of the message that doesn't contain tokens for data (you need to copy @author_name and @date into your translation so the text makes sense, don't translate "author_name" or "date", leave them in English as they are!). 

Dates are special formats in Drupal, with special translation options. 

You can translate Date formats by going to Configuration > Regional and language > Date and time formats (/admin/config/regional/date-time). 

The one being used for "Submitted by" text is "Default medium date". Choose Translate in the drop-down next to Edit button. 

Here you only configure how dates are usually structured in your language (maybe something like D, d/m/Y - H:i in your case for the Default medium format, to have the day before the month). 

Month and Day name translations will probably already be available from Drupal Localization project, but if you see they aren't translated you can translate them yourself through the User interface translation page. 

If you don't see some of the options mentioned here, make sure you have enabled all translation modules, including "Configuration translation".