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According to Japan’s traditional calendar, September 18-22 is known as Tsubame-saru, or “When the swallows leave.” Swallows are very comfortable around human dwellings, and it is common in Japan to find their nests built under the eaves of people’s homes, on the outside of office buildings, or even in the middle of train stations. By autumn, the chicks have grown large enough to fly away on their own.
The National Diet Library has more in-depth information about the traditional Japanese calendar, its origins, and the relationship to the modern-day calendar. To learn more, please visit their site: https://www.ndl.go.jp/koyomi/e/