In the higan (the equinoctial week) of spring when red and pink flowers of peony bloomed, people put red azuki-an for gods or Buddha or ancestors; the color and the shape of those azuki-an reminded people of the peony flowers, therefore, the mochi with azuki-an made in spring began being called botamochi.
牡丹の花が咲く季節、すなわち春の彼岸に、神仏や先祖への供物とされた小豆餡の様子を、牡丹の花に見立てたことから。