In Thessaloniki, the Greek National Tourism Organization (GNTO or EOT) central regional office on Tsimiski is open year-round from 8:30 to 3 on weekdays, and 8:30 to 2 on Saturday. Opening hours may be longer in summer but are not guaranteed. There's also a branch at the airport.
Greek National Tourism Organization. Tsimiski 136, at Dagkli, Kentro, Thessaloniki, Central Macedonia, 54621. 2310/254839; 2310/252170; www.visitgreece.gr.