Your tree has physiological responses to the heat of Summer. It goes into a semi-dormancy to cope with high temperatures, scorching sun, and preparation for Fall.
Protecting your tree during Summer is of utmost importance. Almost every tree should be shaded from afternoon sun, especially in the Summer.
If you miss watering even for a day you may lose trees. This is not the time to be skipping watering, however it happens to be the time for vacation. If someone is coming to water my plants I tell them which plants to water and watch them do it without any instruction first, then I write down everything they have to water and give more specific instructions like "Water it twice as much as you think you should." It also wouldn't hurt to have an automatic sprinkler to give it another watering depending when they plan on watering.
Once the temperature reaches the mid 90s your tree will go into a dormancy and cease to photosynthesize in order to prevent significant water loss. Your tree uses water as a cooling agent while transpiration and photosynthesis cease. Watering your tree with scorching hot water will not aid the tree in cooling itself. Make sure to run water out so you water with cool water.
Certain species of trees can be damaged by leaf burn from the Summer sun. This happens commonly with Japanese Maples among many others. Assuring your tree is shaded from the afternoon sun will greatly aid in reducing leaf burn and overheating problems. It is also common to experience leaf burn on leaves that have no hardened off yet and don't have a cuticle to protect the leaf from the sun.
Root damage is also common if the pot is not protected from the sun. Black plastic pots, clear glass pots, or even your average bonsai pot can overheat and scorch the roots. Other than a shade cover you can do other things to prevent this by shading pots with foliage from other trees, grouping them together, laying some type of screen over the pot that still allows air to pass through (Window screen, etc.).
It is said that some tropical and mediterranean trees like to be repotted in the Summer, but personally I wait until the temperatures reach 50+ degrees regularly and then I repot. This happens to land somewhere between Spring and Summer. I have repotted Ficus, Olive, Schefflera, and Portulacaria Afra in later Summer without any problems, but prefer to do so earlier in Summer to allow a longer window for recovery throughout Summer.