I can't seem to find an end date for the event. If anyone knows please tell me.

The reason is that I've been putting off obtaining some of the accessories because I want to wait until the effects on my current ones run out but I don't want to wait too long and have the festival end without notice.

Hello Hiashi. You can check the girl npc beside the portal to the Onigashima festival. She's named like Guide okuni or something. I haven't been online lately so I forgot, I think the event ends on mid-September.

Yeah you're right. She says it ends on September 23.
Though CS could have mentioned it on their website as well.