When does the oak tree drop its leaves?

Trees are plants. In spring, like other plants, trees grow and make new leaves. In summer they are full of leaves. In the autumn some trees lose their leaves. They are called deciduous trees. Deciduous trees look very different in the winter. Some trees keep their leaves all tear round. These are called evergreens. Evergreens don't lose their leaves. Why do some trees change? When do trees grow?