Gregorian (western) one is an official calendar in Turkey. Sunday is holiday; Friday is Holy day in Turkey. 

Religious Holidays: Turkish people celebrate two religious festivals according to the Muslim lunar hegira calendar which is about 11 days shorter than Gregorian one. Because of this, religious festivals occur 11 days earlier each year. Religious festivals’ periods are public holidays in Turkey. There is also a half day vacation for preparation for festivals. Many shops and offices are close about noon. In religious holiday period, hotels, flights into Istanbul can be very busy with people from all around Turkey and especially roads are full of locals. So you should know when they occur and how they affect travel.

- Ramazan (seker) Bayrami: It is a religious holiday for 3 days. Banks, offices etc are closed but hotels, buses, trains and planes are heavily booked during 3 off days. At the end of the fast of Ramadan, sweets are eaten to celebrate it. Fasting means letting nothing pass the lips for Muslims at day time during 30 days in Ramadan. Although many people in Istanbul observe the fast, most restaurants, cafes etc open to serve all day. Seker means in English candy or sugar. After Ramadan, children go door to door asking for candy and everyone makes social phone calls. 

- Kurban Bayrami: This is the most important religious holiday for 4 days in Muslim countries.Kurban Bayrami means in English sacrifice Holiday. The head of the household sacrifices a sheep and much of the meat goes to charity. It is busier than candy festival and banks offices are closed.

Secular Holidays: Turks celebrate 5 secular public holidays in a year and banks, offices and all government services close. January 1- first day of the new year, April 23 – National Sovereignty and Children‘s day, May 19 – Youth & sports day, August 30 – Victory Day, October 29, Republic day.