I didn't know, either, but Wikipedia says:

https://en.wikipedia.org/wiki/Blue_Monday_(date)

"Blue Monday is a name given to the Monday of the last full week of January, purported to be the most depressing day of the year as part of a 2005 publicity campaign by Sky Travel. The concept is considered pseudoscience, with its formula derided by scientists as nonsensical."

We didn't feel blue. In the United States, Monday was Martin Luther King, Jr. day, which I generally find uplifting as King was one of the great heroes of our time--but even more so because our kids had to work (too bad) but that meant we got to have the grandchildren visit with us for the day (excellent!)

what's the mean of "blue" Monday

Sorry Lisa, but January 18th was Sunday.:)