Palo Alto's climate is the gentlest in California — daytime highs hovering around 20°C for most of the year, almost no humidity, and dry summers. There is no bad month to visit. Spring (March–May) is the best time for wildflowers on Foothills Park and the hills above town; late September into October delivers the year's warmest, clearest weather, though wildfire smoke is a risk most years.
Avoid the week of Stanford commencement in mid-June if you want quiet hotels; that and big-game football weekends in November are the busiest times. The Stanford Theatre closes for several weeks in late summer.