The best time to visit Greece is late spring and early fall. In May and June the days are warm, even hot, but dry, and the seawater has been warmed by the sun. For sightseeing or hitting the beach, this is the time. Greece is relatively tourist free in spring, so if the beach and swimming aren't critical, April and early May are good; the local wildflowers are at their loveliest, too. Carnival, usually in February just before Lent, and Greek Easter are seasonal highlights.
July and August (most locals vacation in August) are always busy—especially on the islands. If you visit during this peak, plan ahead and be prepared to fight the crowds. September and October are a good alternative to spring and early summer, especially in the cities where bars and cultural institutions reopen. Elsewhere, things begin to shut down in November. Transportation to the islands is limited in winter, and many hotels outside large cities are closed until April.