The calendar pages in “When It Happens” symbolize the time Mrs. Burridge feels she is wasting. Calendars are objects that track time, and Mrs. Burridge’s page-a-day calendar condenses the span of a year into a single, disposable space. Presumably, at the end of each day, Mrs. Burridge rips off a calendar page to reveal the next one. She then uses the discarded calendar pages to complete menial tasks, such as writing her grocery list. Throughout the story, it is revealed that Mrs. Burridge is dissatisfied with her life; she feels that she has fallen into a repetitious and empty state of being. Similarly, she treats the calendar pages—symbols of the passage of time—as though they are completely expendable.
The calendar is also significant because it is a gift that Frank gets her every year. Therefore, it not only represents the passage of time, but also a sense of repetition within that time. Mrs. Burridge is uncomfortable with the recurring patterns of her everyday life and the calendar is a symbol of the unvarying nature of her existence. Each passing day for Mrs. Burridge is always the same, so much so that she daydreams about an apocalyptic event that would thoroughly rupture the temporal patterns of her life.
However, instead of counting down to a doomsday scenario, at the end of the story, the calendar is still just scrap paper. Mrs. Burridge uses it to write her grocery list and nothing more. The calendar never actually represents a countdown to extinction, then, though it is still a memento mori—a constant reminder of the inevitability of death—for Mrs. Burridge.