Blue Monday is a false calculation based on things like the gloomy weather, post-Christmas debt, disappointment from not keeping new year's resolutions, dissatisfaction about going back to work and general doom and gloom. Since then, it has become a rather tedious yearly PR event, often designed to promote things that are vaguely linked to improving our wellbeing, more often than not with a complete lack of evidence. No actual scientific studies have ever backed up any claims about Blue Monday.
However, we live by the calendar and we need more mental health awareness.
You can feel low at any point in the year. When it happens, please make time for yourself and seek great advice on how to manage symptoms - https://www.mentalhealth.org.uk/