Why is Shirley Jackson purposely vague about where "The Lottery" takes place?
"The Lottery" is an allegorical story that may be read in various ways, as a criticism of patriarchy, mindless violence in society, the darker side of tradition and ritual, or the capitalist social order. This broad scope is created by ambiguity in concrete details; anchoring the narrative too firmly in the real world would limit its power and curtail its range. (The precise date given in the first line of the story prevents it becoming too vague.)
In fact, the village in question is modeled closely after the settlement where Jackson was living at the time, North Bennington, Vermont. Jackson explained,
I hoped by setting a particularly brutal ancient rite in the present and in my own village, to shock the story's readers with a graphic dramatization of the pointless violence and general humanity in their own lives. ("The Lottery: Historical Context")
Given the general tenor of the story, any precise reference to where it was supposed to have taken place would also have been irresponsible. Jackson later commented,
....what [readers] wanted to know was where these lotteries were held, and whether they could go there and watch. ("On the Morning of June 28, 1948, and 'The Lottery'")
She used a number of details from life in her home town to make the narrative vivid and concrete, but if she had gone further and supplied the town's name, she would have turned the story into a slanderous attack on her neighbours, at least by implication.