Bonsai trees, like all trees and plants, react to the change in seasons. If you're keeping a bonsai tree outside, it will have an even stronger reaction to the change in temperature, sunlight, and the amount of rainfall in the region. In some regions there are four distinct seasons, and in others the seasonal changes are more subtle. In any case, understand the way your tree species reacts to the seasons in your region, and let that information guide the way you care for it.  Trees lay dormant during the winter; they aren't producing leaves or growing, so they don't use as much nutrition. During this season, watering the tree is about the only care it needs. Avoid trimming it too much, since it won't be able to replace the depleted nutrients until spring. In spring, trees begin using the nutrients they stored during the winter to sprout new leaves and grow. Since your tree is in transition during this time of year, it's a good time to repot the plant (adding extra nutrients to the soil) and begin trimming. Trees continue to grow during the summer, using up the rest of their stored nutrients. Be sure to water yours well during this period of time. In the fall, tree growth slows, and the nutrients begin accumulating again. This is a good time for both trimming and repotting. Your bonsai tree's light needs depends on the species and your climate, but most will thrive in a location that receives morning sun. Turn the tree 90 degrees every few days so all of the tree foliage can receive an equal amount of light. Indoor trees may need a light shade cloth over the window during hot, bright summer months. During the summer, it's fine for the tree to spend the majority of time outside. Bring it inside overnight when temperatures dip below about 40 °F (4 °C). In preparation for the winter, get your tree acclimated to spending more time indoors by moving it inside for a few hours at a time, and increasing the time it spends indoors every day until you bring it inside entirely. Fertilize the tree with a special fertilizer meant to keep bonsai trees healthy. When the soil begins to look dusty or dry, water the bonsai. The exact watering frequency depends on the tree species and the season. You may need to provide a little water every day during the summer, but only water once every few days during cold winter months.

Summary:
Pay attention to the seasons. Give the tree morning sun and afternoon shade. Protect the tree from extreme temperatures. Provide food and water.