Blue Monday means of small value
Blue Monday has the meaning of a short time. Often it is used when someone has had a short-lived relationship with someone: that relationship has only lasted a blue Monday. It is not quite clear where the term comes from: the one says it comes from Lent, the other of a simple drunkenness, or the dyeing of wool, to the dust of a volcanic eruption. It is not clear, but it is agreed that this is a short period of time. A blue Monday should not confuse you with Blue Monday, because that is something else.