SLO has one of the more consistently pleasant climates in California. The coast and the hills work together to moderate temperatures — marine fog rolls in overnight, burns off by late morning, and the afternoons are warm and sunny. Average highs run from the low 60s Fahrenheit in winter to the mid-to-upper 70s in summer. Actual extremes are rare.
The catch is fog. Summer mornings — particularly June and July — can be grey and cool until 11 AM, a phenomenon locals call "June Gloom" (though it runs well into July). If you are visiting specifically for the sun, September and October are the standout months: the summer crowds have thinned, the fog is mostly gone, and the temperatures are excellent. October is often the warmest month of the year.
Spring (March–May) is green, pleasant, and quiet. Wildflowers bloom along the hills and in Montana de Oro State Park south of town. Winter is mild enough that visiting is still worthwhile, though rain arrives mostly between November and March.
The Thursday Night Farmers Market runs every week year-round, rain or shine. If your visit coincides with it (any Thursday evening), it's worth organizing your schedule around. The Farmers Market draws visitors but it's primarily a local event, and it gives a genuine read on the town's character.
Cal Poly's academic calendar matters if you care about crowds. During summer the student population drops significantly and downtown is noticeably quieter. Homecoming and graduation weekends (typically mid-June) fill accommodation quickly.