I faint but do not know why. It happens only once a year, usually in june and have been for tests for diabetes, neurological, gastrointestinal etc. All doctors will say is its avaso vegal syndrome but no one can tell me the cause and why it only happens

Faints. Keep a diary of everything you eat and what you do in june. Look for some re-occuring activity or food eaten. Perhaps you have an allergy to something blooming in june. This is difficult to tract down but keeping a journal of events may spot the culprit.
Difficult.. Symptoms which occur very seldom are quite hard to track down! . Once a year is...Odd. That means the usual cardiac problems can be ruled out. Makes me wonder if you have some activity in which you engage in june, one which causes syncope. Could even include activities which stimulate your carotid artery and cause a reflex faint. Best of luck!