The next days you will see a lot of yellow banners and signs with chinese style Thai characters in red. They announce the vegetarian week and let you know that this place is offering vegetarian food. I was suprised how many businesses are participating. Seacon Square for example is advertising vegetarian dishes, and when I drove along Srinakarin road, I noticed a lot of restaurants with the "เจ"-banners. Even in my street, Soi Wat Dan Samrong, I saw a sign (as shown in the picture).
Richard Barrow was live-tweeting from the BTS National Stadium where the festival is celebrated as well, even with some free dishes as far as I understand.