The best time to visit Patmos is May–June or September–October. The weather is warm, the sea swimmable, the ferries running, and the island at a civilised level of occupation. July and August bring more visitors (though Patmos never reaches the saturation levels of Mykonos or Santorini), higher prices, and heat that makes the midday climb to the monastery uncomfortable.
Easter on Patmos is one of the great Greek Orthodox celebrations: the midnight service at the Monastery of Saint John, with candles and incense and the chanting carrying over the hillside, is a genuinely moving experience. The island fills with pilgrims for Holy Week and reservations are essential.
Winter sees the island quiet to the point of closure: many restaurants and shops close, ferry services reduce, and the village life shrinks back to its year-round residents. The monastery remains open, the weather is mild and often sunny, and the whole island takes on a different character — peaceful rather than empty.