Meat free week was an initiative launched in Australia, which was also picked up by the US and the UK. The reasons behind this proposal are many, and include the high amount of water and grain needed to breed cattle in comparison to that needed to grow vegetables and vegetable sources of protein. Read more about pulses and sustainability.
Many organizations and celebrities encouraged people to not eat meat, including fish and sea food, for the last week of March. Initially Sir Paul McCartney boosted the Meat Free Monday campaign and now there are plenty of celebrities advocating not necessarily for going completely vegetarian but for reducing meat consumption, like Gwyneth Paltrow, Emma Thompson, Moby and famous chefs including Jamie Oliver.
But let's stop talking about it, and instead let's hear directly from Simon Bryant, an Australian Pulse Advocate, on why this campaign is so important. His interview on the National Broadcaster in Australia, ABC, starts about 2 minutes into this recording: http://www.abc.net.au/radio/programitem/perwDOy7jQ?play=true.