Monday Blues

People likes to say “Monday Blue, Monday Blue”, but what is actually the blue Monday defined for? what is the meaning of BLUE? Why is blue Monday but not yellow, red or green?
From what i get from internet, blue always meaning the melancholy pertaining to mood, situation and feeling.

In most of the city, Monday is the 1st day of working after an exhilarating weekend. Monday, people have to back to work, student needs back to school, and everyone needs to attend the viscous workload. Oh god, Monday is definitely BLUE.

However, we can also define Monday Blue in a positive and spiritual way. Blue present natural, relax, stress-less, new start. Monday is a brand new day, new start, forget about the sadness, hurts, failure, and all the badness. Oh my lady GaGa, Monday is so BLUE!