Blue Monday
/ˌbluː ˈmʌndeɪ/ /ˌbluː ˈmʌndi/noun
Originally in German contexts. A Monday during which people choose not to work, especially for a celebration, or because they have indulged excessively over the course of the weekend. Now chiefly historical.
Origin
Late 18th century. From blue + Monday, after German blauer Montag Monday on which journeymen were not required to work, holiday Monday.