It is so hot here this weekend. Normally in June this area of Oregon should average about 78° for the high. This weekend we're expected to hit 100°+. With it being so hot, you've got to be certain to take care of your plants so that they make it through such high temps. That is especially true if you go from lower temperatures to really high ones like we've done (we were in the 60s).
If you're growing in pots, you may find you need to water multiple times throughout the day. I try to water in the morning, check them in the afternoon (water if necessary), and then give them another watering in the evening. Typically if you've gotten that hot, you're also going to be hot at night and the plants will need more water to get them through the day and night than normal.
If you can, think about setting up a sprinkler hose, soaker hose, etc through your garden. I picked up two 50' hoses last night to do exactly that, but realized with my back limitations that I can't do it on my own. So my husband will have to help me later tonight or tomorrow morning to hook it up. This allows for a constant trickle of water to your plants. This will better help them get through the hot days than just a bunch of water once or twice a day. A sprinkler hose has little holes in it that can either be directed down to the ground or allowed to spray things. A soaker hose is made of a porous material that allows the water to seep through.
Another thing you might want to do if your plants are in containers is move anything that isn't ready for all that heat and sun into the shade. I planted my cucumbers a week ago, but they are not ready for all of this. As such, we moved the containers onto the patio where they will get more shade.
When watering, try not to get too much water on the leaves. While it may evaporate quickly, it can also act like a magnifying glass on your leaves while it's still there, burning the leaves.
If you have plants like lettuce, broccoli, spinach, etc, don't be surprised if they bolt in the heat. My lettuce was big and fat and now suddenly because of all the heat it is bolting. This is the process where the plant puts up a shoot that will have the flowers and seeds in it. It's also referred to as going to seed. You can try to delay this process by keeping them shaded, but once the process starts your plants likely don't have a lot longer.
If you're in an area that gets a lot of extreme sun and heat, you might want to consider putting up a sun shade. They come in various strengths, which block out some of the sun and heat. When we get into a house I'm planning to set one up to put my greens, chinese cabbages, spinach, etc in so that they last longer and don't bolt.
Hope you don't have to deal with too much extreme heat this summer, but if you do, I hope these tips help.