In the temperate Pacific Northwest, you can plant any time of year as long as the ground is not frozen. Most gardeners will recommend planting in the Fall when the Autumn rains are prevalent and during the coolness of spring. During the hot summer, it is still ok to plant as long as you plan on being around to water. New plantings need a little more care and more frequent watering.
Similarly, it is ok to plant during our winter as long as the ground is still soft and workable. Tender plants may need a healthy application of mulch to help them through the winter.