“Meatless Monday” is an international campaign that encourages people to not eat meat on Mondays to improve their health and the health of the planet.
Meatless Monday focuses its initiative on Mondays for several reasons. Friday is traditionally already a meat-free day among Catholics and Orthodox. Monday is typically the beginning of the work week, the day when individuals settle back into their weekly routine. Habits that prevailed over the weekend can be forgotten and replaced by other choices. A weekly reminder to restart healthy habits also encourages success.
A 2009 trial published in the American Journal of Preventative Medicine provided individuals with weekly health prompts and encouragement. Approximately two thirds of participants responded with improvements in their overall health, eating habits and physical activity levels. -Wikipedia