Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Orlando to Thessaloniki as you normally would.
Due to multiple factors, expect to potentially pay more when booking in June. For low rates, November can be a great choice for travel to Thessaloniki. Flights to Thessaloniki from Orlando in November are about $1,092 on average during November, but can be found for as low as $425.