Mondayitis
/ˌmʌndeɪˈʌɪtɪs/ /ˌmʌndiˈʌɪtɪs/informal and humorous
noun
Reluctance to attend school or work, or a reduction in working efficiency, experienced on a Monday morning.
Origin
Early 20th century; earliest use found in Weekly Advocate (Victoria, Texas). From Monday + -itis.