While Shirley Jackson does provide very subtle foreshadowing, there is certainly ambiguity in her narrative about the lottery, a function that only takes a short while:
[It] took less than two hours, so it could begin at ten o'clock in the morning and still be through in time to allow the villagers to get home for noon dinner.
Throughout the narrative, a third-person objective point of view is used, and, while details are provided, there are no judgments given, lending ambiguity as to what is to occur. Certainly, Jackson disarms her readers with the bucolic scene of the men of the village talking of "planting and rain, tractors, and taxes." Along with this description, Mr. Summers, "a round-faced, jovial man" who waves and calls out, "Little late today, folks" as though everyone has assembled for a picnic or fair does not appear to be someone who would conduct anything sinister. Most importantly, the word "lottery" is ambiguous of itself, for it is never actually defined, and, thus, the gathering of the villagers seems harmless.