How much weight you put on this is up to yourself - its only the Turkish press that's reporting it so far. Nothing from any of the major embassies or consulates, at least those I can understand. Anyone on here that reads Hebrew may want to check the Istanbul Israeli consulate site.
"Turkish police issued a confidential warning to provincial security directorates across the country’s 81 provinces, cautioning against imminent attacks by the Islamic State of Iraq and the Levant (ISIL) during Youth and Sports Day celebrations on May 19, security sources have stated."
I'm not going to list all the sources, just google the thread title and you can read them for yourselves.
Be safe this coming youth and sports day CBF folks (and everyone else, obviously).
JF