It could very well be November 17th, which has had some of its members arrested. The Ambassador's office is on the main street side on the top floor to the left of the Great Seal in the front of the building. More than likely, it was aimed at his office. The Greek State Opera House is located next to the Embassy.
http://www.emergency.com/nov17rpt.htm
"The group's last known attack was made in May of 1995 when it fired two anti-tank rockets at a Greek television network building. The television network was called a capitalist tool. There were no injuries in the attack."