The spring and the fall are the most common times to plant olive trees, though they can be planted throughout the summer if the conditions are right.
The advantage to spring planting is that the trees have all summer to grow into their site before the winter cold sets in. In colder climates or when planting small (one gallon) trees, spring is the best choice. Timing a springtime planting can be tricky depending upon the rain. There will be a narrow window when the soil has warmed up and dried out enough to plant but not turned rock-hard yet. This window of moist-but-not-wet is the optimum time to plant.
In warmer winter climates, autumnal plantings have the advantage of being in the ground before all the rain comes and settles them in for the winter. The trees can rest for the winter and are ready for growth as soon as the air and soil warm in the spring
Summer plantings have to be timed correctly. Do not plant before a heat wave or without being confident that the irrigation system is in working order.
In general, winter planting is not a good idea because the soil is too wet. If the wet soil is worked, it will turn to slippery mud and all the pockets of oxygen will be eliminated, leaving a heavy, oxygen-less vault for the roots.