Because it is a day that is the most important event in Buddhism 3 events is the birthplace of the Lord Buddha and the Buddha's enlightenment by all three events have occurred at the lunar month or the full moon day of the month 6 wisakha.
It is similar to the most important events in Buddha three events, namely the birth, enlightenment and death of Buddha. The three incidents have occurred at the full moon of the 15th lunar month, or six months Visakha.
Because it's like the day of the incident is the most important in Buddhism 3 events together is the birth, enlightenment and parinirvana of the Buddha. By three events occur at night the moon, 6 15 days