The stamp on the cover is part of a 4v set issued 07 Feb 2008, which commemorated the 800th anniversary of Nasreddin Hodja's birth, according to http://trphilately.blogspot.com.
More on Nasreddin Hoca and Turkish folklore:
The tradition of folklore—folktales, jokes, legends, and the like—in the Turkish language is very rich. Perhaps the most popular figure in the tradition is the aforementioned Nasreddin (known as Nasreddin Hoca, or "teacher Nasreddin", in Turkish), who is the central character of thousands of jokes. He generally appears as a person who, though seeming somewhat stupid to those who must deal with him, actually proves to have a special wisdom all his own:
One day, Nasreddin's neighbor asked him, "Teacher, do you have any forty-year-old vinegar?"—"Yes, I do," answered Nasreddin.—"Can I have some?" asked the neighbor. "I need some to make an ointment with."—"No, you can't have any," answered Nasreddin. "If I gave my forty-year-old vinegar to whoever wanted some, I wouldn't have had it for forty years, would I?"
The postmark reveals that this cover was sent from Heykel, Bursa, Turkey's fourth largest city, on 26 Apr 2008.