You can install a sod lawn anytime during the year when the ground can be tilled and irrigated. There is no need to wait for a certain season to put in a new lawn.
Is installing sod better than seeding?
Yes, sod takes less work than seeding and requires less water. Most seeded yards will need re-seeding or over-seeding when seeds do not germinate. Birds, soil erosion and wind will take their toll on seed as well making the job very frustrating. A seeded lawn requires months of nurturing and additional water before it is established and useable. A sod lawn will be ready for use within a few weeks and will be green and attractive almost immediately after installation since it is already mature grass.
What if I have existing, unwanted or dead grass?
Remove all old grass by either renting a sod cutter or by hand using a shovel. Once you are down to dirt prepare your soil as suggested here.
Can I just rototill the dead lawn into the dirt?
This is not advised for several reasons. The tiller will have a hard time breaking down the existing root system leaving you with very clumpy ground to work with. The existing roots can also re-grow and create areas of unwanted grass in the new lawn. If you choose to go this route be sure to spend the time necessary to rake out all the large clumps and remove as much of the root material as possible, make the ground as smooth as possible. Remember, the better your soil preparation is the better your new sod will look.