Today I found this in Thissio Station.
It says that ISAP (Metro No. 1, Green Line) runs only from 10:00 to 16:00 on the 29th June, the upcoming general strike day. We won't know for sure until tomorrow afternoon, but it looks highly probably.
Besides I read a banner in Piraeus port that the ships will not leave it on the same day. This is less sure, but evidently the port workers (note, however, that some say that the workers who strike are a minority and the majority is hindered by the active union members) want to strike.