Is this fequently used to mean the depressive
day after the weekend?
Here's what Brande & C provide:
Blue Monday, (a) a Monday following a Sunday of dissipation, or itself given to dissipation (as the Monday before Lent). (b) a Monday considered as depressing because it is a workday in contrast to the relaxation of the weekend.
In my dialect, (b) is more common than (a).