Soil is a heat sink at the surface and it has a constant temperature at a specific depth. The sun and ambient air temperature warm the upper few inches of the soil in the summer depending on if the soil is shaded or covered with mulch. The cold air and ice of winter cool or even freeze those upper few inches. Below the upper few inches the soil temperature doesn't change as rapidly as the upper few inches with weather changes, but it will change eventually. It depends on how long the ambient temperature is either hot or cold. Going farther into the soil, the temperature doesn't change year round. Since heat rises, the lower soil will eventually warm the soil above even in cooler weather.
It seems to work best if you can cause your plants to develop deep roots to take advantage of the more available minerals, moisture, and moderated temperatures. With deep roots, many plants will appear to wilt in the hot summer sun. They can not move enough moisture through the plant tissues to replenish the moisture lost through the leaves due to the extreme heat. Overnight, with cooler air and lower transpiration through the leaves, the moisture content recovers and the leaves usually look normal in the morning.
I always attempt to plant transplants deep to take advantage of the more constant soil temps and more constantly available subsurface moisture. By limiting the amount of moisture and mineral nutrients I apply from the surface of the soil, the roots tend to develop deeper in search of moisture and nutrients. They also remain cooler in the hot summer sun.