Jeddah has long summers lasting 8-10 months, usually from March to September. Being a coastal city the humidity in summers rises as high as 98% therefore it’s not advisable to visit it in summers especially in June and July when the temperature rises on average to 40 degrees and nights are 30 degrees. Winters in Jeddah are much more pleasant. They last from November to March and the calm tropical winds make it enjoyable for tourist to visit the city. Therefore if you don’t like the heat it is best to visit Jeddah in winters.
If you are planning to go to Jeddah for Hajj purpose, check the lunar calendar for the month of Zilhajj. For Umra the best time is Ramadan when the atmosphere is best for religious retreat.