Mandala of Sixteen Great Arhats
"At the time of the time Buddha's parinirvana, sixteen great Arhats vowed to remain in the world to protect the Dharma until the time of the next Buddha, Maitreya. Their forms were replicated by sculptors in China, where the Arhats appeared to the T'ang emperor T'ai-tsung, and honored as symbols that the Arhats support all who sincerely desire to follow the Dharma.
In Tibet, images of the Arhats became the focus of prayers and practices that strengthen virtue and remove obstacles to realization. All who visualize the Arhats as gathered around the Buddha gain great merit and quickly develop insight into the Buddha's teachings.
The days of the new moon and the tenth, fifteenth, and twenty-fifth day of the lunar month are traditionally auspicious times for this practice." -back description