In Mahayana sutras， a metaphor for truth or reality: the moon is reflected in all bodies of water （lakes， ponds， etc.） while its brilliance is undiminished.
"Many people would like to know the religious significance of the full moon and new moon. To Buddhists， there is a special religious significance especially during a full moon because certain important and outstanding events connected with the life of Lord Buddha took place during a full moon. The Buddha was born on a full moon. His renunciation took place on a full moon. His Enlightenment， the delivery of His first sermon， His passing away into Nibbana and many other important events associated with His life-span of eighty years， occurred on full moons. Buddhists all over the world have a high regard for full moons. They celebrate these days with religious fervour by observing precepts， practising meditation and by keeping away from the sensual worldly life. On such days they direct their attention to spiritual development. Apart from Buddhists， it is understood that other co-religionists also believe that there is some religious significance related to the various phases of the moon. They also observe certain religious disciplines such as fasting and praying during full moon days. ... It is believed that the moon， like the planets， exerts a considerable degree of influence on human beings. It has been observed that people suffering from mental ailments invariably have their passions and emotional feelings affected during full moons. The word 'lunatic' derives from the world 'lunar' （or moon） is most significant and indicates very clearly the influence of the moon on human life. Some people suffering from various forms of illness find their sickness aggravated during such periods. Researchers have found that certain phases of the moon not only affect humans and animals， but also influence plant life and other elements. Low-tides and high-tides are a direct result of the overpowering influence of the moon. ...
In view of the possible influence of the moon， the ancient sages advised people to refrain from various commitments on this particular day and take it easy for the day. They are advised to relax their minds on this particular day and to devote their time to spiritual pursuits. All those who have developed their minds to a certain extent can achieve enlightenment since the brain is in an awakened state. Those who have not trained their minds through religious discipline are liable to be subjected to the strong influence of the moon. The Buddha attained His Enlightenment on a full moon for He had been developing and attuning it correctly for a long period. In days gone by， full moons and new moons were declared public holidays in many Buddhist countries and people were encouraged to devote their time to spiritual development. It was only during the colonial period in Asia that holidays were switched over to Sundays. In view of this， some Buddhist countries are now trying to re-introduce the former lunar system of holidays. It is advisable to observe full moons as a religious day to concentrate on peace and happiness by calming down the senses. Many Buddhists observe the eight precepts on full moons， to be free from family commitments and to keep away from worldly pleasures in order to have peace of mind for their spiritual development. . When we consider all those occurences， we can understand why our ancestors and religious teachers have advised us to change our daily routine and to relax physically and mentally on full moons and new moons. The practice of religion is the most appropriate method for people to experience mental peace and physical relaxation. The Buddhists are merely observing the wisdom of the past when they devote more time to activities of a spiritual nature on New Moons and Full Moons."