San Jose has a Mediterranean type climate with distinct wet and dry seasons. The rainy season normally happens from November until March when 82% of the annual rainfall falls. During dry season (April - October) expect consistent, warm, sunny days. Don't expect snow in San Jose, there are only a few documented occurrences of it.
When you decide to visit San Jose depends on what you want to see. While many tourists prefer the dry season because of the climate, you can expect to pay more at this time of year as well. If you don't mind a bit of rain go in the early stages of the wet season. This way you avoid crowds and spend less on your holiday.