The Summer Solstice is on the 20th or 21st June in Turkey as it is throughout the rest of the world. The actual day varies annually. So, the longest day is either the 20th or 21st June. Sunset in Istanbul on the 21st June 2017 would have been around 21.00 and roughly the same in Calis.
The shortest days are around the 21st and 22nd of December.