The Thais who have Thai-Chinese ancestors celebrate Vegetarian Festival in every September-October. It is a bit weird that the whole festival is named Vegetarian Festival (tesagan gin jay in Thai), because it involves so much more than food.
I´m not going to try explain the whole religious origins of this event because there are many different interpretations of them and because the explanation would be long, complex and I´m not even familiar with all that confusing stuff.
The food during Vegetarian Festival should be vegan, though you can not be sure if all the sellers know it exactly. Some people think the word jay means vegetarian food that can include milk and eggs, but some people say it is pure vegan diet. Well, at least in 7-Eleven shops they sell vegan versions of some of them pastries that would otherwise include milk and egss.
Jay food has some other principles than being just vegan or vegetarian. Jay food should not include garlic or different onions and neither be too spicy. In Thai jay vegetarian restaurants I´ve noticed that the food is sometimes a bit bland, but I haven´t paid attention to it being onion free.
The Sino-Thais seem to be very entrepreneurial people and during Vegetarian Festival there are numerous jay food stalls. In Bangkok I found them expecially in the Chinatown (Yaowarat) and especially in Talat Noi area there. In Chiang Mai I found them at the Thapae Gate.
The Festival has become something that looks a lot like food carnival because Thais love to celebrate and eat. The food stalls during the Festival include the familiar dishes that can be found in the usual jay restaurants in Thailand: curry sauces, fried vegetables, noodle soups and mock meats. But there were especially many stalls selling deep fried food: dumplings, gyozas, spring rolls and tempuras. Maybe that is because all that cooking oil makes the otherwise bland food tasty and maybe it tells about the Chinese origins of the festival.
Other things than food during the festival
The attendants of the Festival dress in white, wash themselves properly, do not have sex, do not swear and, of course, eat only jay food.
Because this is a Thai religious celebration you might think that there would be chanting monks dressed in orange. However, this is not the case, because these is not a Buddhist festival. Instead of buddhist wats people gather in Chinese temples.