This is what the Wikipedia entry on Meatless Mondays says:
"Meatless Monday is a non-profit initiative of The Monday Campaigns Inc. in association with the Johns Hopkins School of Public Health Center for a Livable Future. The program follows the nutrition guidelines developed by the USDA. Meatless Monday is part of the Healthy Monday initiative. Healthy Monday encourages Americans to make healthier decisions at the start of every week.
Meatless Monday focuses its initiative on Mondays for multiple reasons. Monday is typically the beginning of the work week, the day when individuals settle back into their weekly routine. Unhealthy habits that prevailed over the weekend can be forgotten and replaced by positive 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."
And PLEASE check out the Wikipedia entry on the Environmental impact of meat production.
This is just a little something my family and I have decided to partake in, doing what little we can. And there are tons of meals you can have that are meatless! We've had Veggie Stir-Fry, Pasta Salad, Quiche (we obviously aren't doing Vegan Mondays since we wouldn't be allowed eggs then LOL), and I plan on having fun looking up different meatless meals!