Santa Monica's climate is famously mild year-round. Summers (June–September) are warm but not hot — coastal fog, known locally as the June Gloom, regularly blankets the city in the mornings and burns off by afternoon. This makes the beach usable through most of the day, though the fog can be dispiriting if you are not expecting it. July and August are the busiest months for tourism.
Spring (March–May) and autumn (October–November) are generally excellent: clear skies, warm temperatures, fewer crowds. December through February is mild by most standards — daytime temperatures in the 60s Fahrenheit — and the beach and restaurants remain open and usable, just quieter.