European Night of Museums 2019

http://msurs.net/index.php/en/dogadjaji-6/noc-muzeja/98-evropsk-noc-muzej-2015-godine-3

Celebrated every year on the Saturday just before International Museum Day. This year, the European Night of Museums will be held during the night of 18 to 19 May 2019. Created in 2005 by the French Ministry for Culture and Communication, the event inspires museums to open their doors to the public until 1:00 a.m. and free of charge. By welcoming the public at night, museums provide visitors with a new perspective on their collections, applying a more creative and sensory approach.
source: ICOM
Here is a list of free museums in Turkey:
https://arkeofili.com/18-mayis-aksami-muzeler-herkese-ucretsiz/