;-) you're missing a verb... Since when do you manage this event? or

More correctly (because you are asking a literal question) -- When did you start holding this event?. When did you start holding this event? or Since when have you been holding this event? (perfect tense always trumps the others for unambiguous expression of meaning - perfectly clear).

For English tips, it may not be a good place to learn Take my word for it ;-)