Likely very hot. More so in the interior since along the coast at least you should get some breeze.
Not much different from SoCal.
The only difference is that when you are at home you spend your time in an air conditioned home or office or car, and you don't realize how hot it is because you don't stay outside in the scorching heat that long, just the time to transfer from the car to the office or to the house and viceversa.
But when you are sightseeing in Sicily, you will be spending a lot of time walking outdoors in the city streets from sight to sight, or walking in the scorching hot sun visiting ruins at the archaeological sites, of which there are many in Sicily. Your only time in a cool place will be at your hotel in the evening and night, or maybe in the car, if you rent one (although a car is of little use in Palermo's Manhattan style traffic). During the day you'll be in the outdoor in the heat.
If you want to see what you'll experience in Sicily in July, when it's June, spend an entire day visiting Irvine (outside) on foot, maybe with a small back pack on your back.