A vegan cupcake would be a good way to celebrate, or you could cook a vegetarian meal for dinner. For inspiration check out any of these vegetarian cookbooks.
The 1st October as World Vegetarian Day was started by the North American Vegetarian Society in 1977 and endorsed by the International Vegetarian Union in 1978.
Considering vegetarianism or just want to find out a bit more about what being vegetarian entails? – The Complete Idiot’s Guide to Being Vegetarian might be a good place to start.