The story starts out identifying the date upon which the event is held, June 27. The opening paragraph explains that preparation for the event needs to start earlier in larger villages, and that the event itself even takes longer. "In some towns there were so many people that the lottery took two days and had to be started on June 26th." In the village in which the story is set, however, the entire ritual takes only a few hours.
This event has gone on long enough for parts of the ritual to be forgotten or shortened and for equipment used to need replacement or repair. "Mr. Summers had been successful in having slips of paper substituted for the chips of wood that had been used for generations."
Probably the most important clue is provided by Mrs. Hutchinson's comment to Mrs. Delacroix. Mrs. Hutchinson arrives late to the gathering of the townspeople. As she joins the crowd, she says to Mrs. Delacroix,
Clean forgot what day it was...Thought my old man was out back stacking wood...and then I looked out the window and the kids was gone, and then I remembered it was the twenty-seventh and came a-running.
This makes it obvious that some important event happens on the twenty-seventh of June every year.