The best time to visit Andros is May to June or September to October — the weather is warm, the island is green from spring rains, and the summer crowds have not yet arrived (or have departed). July and August bring reliable sun and the island fills up, especially at weekends when Athenians arrive by ferry, but Andros never becomes as overcrowded as Mykonos or Santorini.
Serious walkers should aim for April or May, when the springs are running, the wildflowers are out, and the stone paths are at their most pleasant. The Museum of Modern Art runs its major exhibition through the summer months and is closed from November to April.
The Meltemi (north wind) blows reliably from July to August and can make sea crossings from Rafina choppy — not dangerous, but unpleasant for those susceptible to seasickness.