history
From the 《妙法莲华经·方便品》 (Lotus Sutra): 如优昙钵花,时一现耳 - 'like the udumbara flower, which appears only once in an age.' In Buddhist lore, the udumbara blooms once every 3,000 years; 昙花 in modern Chinese refers to the night-blooming cereus, which opens for only a few hours after dark.