Setting
Omelas is described to us as a utopia. A place where everything is perfect. The city is located on a waterfront with its Eastern and Southern sides facing the water. The Western and Northern fronts are surrounded by small patches of fields (used for growing food) and then immediately below 18 large snow-capped mountains. Beyond the mountains lies something so unimaginable it may not even exist. However there are other cities throughout the area which people can visit Omelas via train from. Omelas is presumably located on Earth.
To place an exact date on the story would be almost impossible. It occurs near the start of Summer and other than that all we know is that they seem fairly advanced (use of trains) and have knowledge a lot like ours which indicates the setting is probably within the last 200 Earth years.
Due to the start of Summer the story occurs during the beginning of the Summer Festival. This is a grand time of merriment and laughter between the citizens of Omelas.