Translating the Days of the Week through a WordPress filter.

awesome_weather_spanish_days_of_week.php

function awesome_weather_spanish_days_of_week()
{
	return array('Do', 'Lu', 'Ma', 'Mi', 'Ju', 'Vi', 'Sa');
}
add_filter('awesome_weather_days_of_week', 'awesome_weather_spanish_days_of_week');