Ideally, fresh sod should be harvested within a day of its use, and laid down during its growing season--at times of the year that minimize stress on the sod, and encourage all energies and nutrients to be focused on establishing roots in the new soil. For most climates and grass varietals, this translates into spring, early summer or early fall periods. Avoid the peak summer heat and winter frosts. Always lay sod on a firmed layer of rich topsoil, or compost that is evenly moist, but not wet.
Spring is one of the easier times to lay sod, as long as you complete the task well after the last threat of frost. The temperatures are mild, the rain helps with establishment, there is no threat of drying out and the sod will be ready to be walked on by the summer.
The key to a successful installation in early summer is maintaining evenly moist sod throughout its establishment period, and not allowing any patches--particularly the sod's edges and seams--to dry out. Watering every three to four days for a month, or so, will usually ensure this in all but the most warm and arid climates.
Fall is also a good time to lay sod, provided you allow plenty of time for the sod to establish itself before any frosts--at least four to six weeks. Maintain an evenly-moist sod up until the first frost. In areas with cold, winter climates, mow the sod to a low, nearly short height, and apply a lawn fertilizer before frosts come.
While the sod will not use the nutrients over the winter, it will store them in the thatch, roots and soil, giving a boost of greening in the spring when it is too early to fertilize the new shoots.