Tinos has a typical Aegean climate: hot dry summers and mild wet winters. The island is busy in summer but far less crowded than neighbouring Mykonos, which makes it more appealing in July and August than many Cycladic islands.
The major exception is 15 August, the Feast of the Dormition of the Virgin Mary. On this day Tinos transforms: tens of thousands of pilgrims arrive by boat, accommodations fill weeks or months in advance, and the main street from the harbor to the church becomes a river of crawling devotees. It is one of the most moving and overwhelming spectacles in Greece. If you want to witness it, book far ahead. If you don't want the crowds, avoid the week around August 15.
March 25, Greek Independence Day, is also significant — the announcement of independence was timed to coincide with the feast of the Annunciation, and Tinos, with its miraculous icon discovered two years earlier, has always had a particular resonance on this date.
Spring (April–June) and early autumn (September–October) offer the best combination of good weather, fewer crowds, and lower prices.