Monterey has a mild, foggy climate year-round — more temperate than San Francisco but prone to the same coastal fog. Summer (June–August) brings the most visitors and the thickest morning fog, which typically burns off by early afternoon. Temperatures rarely exceed 20°C even at the height of summer, which means you will want a jacket most evenings.
Late spring (April–May) and early autumn (September–October) are the best months: the fog is lighter, the crowds thinner, and the light at its clearest. The bay is full of humpback whales from July through November, feeding on the massive concentrations of krill that upwelling brings to the surface. Grey whales pass on their southward migration in December and January and return northward in February and March — both passes are visible from Point Lobos and Fisherman's Wharf.
Winter is off-season in terms of tourists but not bad for visiting — the aquarium is uncrowded, the adobes are quiet, and storm light on the coast can be spectacular. Rain is most likely from November through March.
The Pebble Beach Pro-Am golf tournament in February closes much of the 17-Mile Drive to non-participants and fills every hotel in the area. The Monterey Jazz Festival in September is one of the oldest and most respected in the country, and fills the town for a long weekend — book accommodation months ahead if you plan to attend.