Oriente Tango Marathon is in Istanbul on 17–20 September 2026. Tangocat lists it under Turkey / Istanbul.
Four days in the city is a long weekend for visitors and several stacked milonga nights for locals. The dates overlap Fethiye Camp; anyone looking both ways should compare calendars now.
Venue and registration are on the source link. Tango News only gives the date, city, and a short frame.