Jeddah is hot. The Red Sea coastal climate means high humidity year-round, and summer (June through September) is genuinely punishing — temperatures of 38–42°C with humidity above 80%. Air conditioning is everywhere indoors; the sun is brutal outdoors.
The right time to visit is November through March. Daytime temperatures in the mid-20s, evenings mild and dry, the Corniche pleasant after dark. December and January are the coolest months — bring a light jacket for evenings.
Avoid the Hajj month (timing varies year to year — it follows the Islamic calendar) if you don't want to share the city with three million pilgrims. The lead-up to Eid al-Adha is the busiest tourist crunch of the year. Ramadan (also moves yearly) is interesting — restaurants open only after sunset, the city stays awake until dawn — but daytime services are limited and eating in public during fasting hours is socially impossible.