Masi refers to the Tamil month. Magham is the Nakshatra that is ruling.
How the date is decided annually
Masi Magham falls on a full moon day in the month of Masi when the Magham Nakshatra is aligned with the moon.
Significance of the festival to Hinduism
This day signifies the descent of heavenly beings to earth. The day provides combined benefits of abundance and prosperity which are associated with the full moon and the majestic quality of the Magham star. This day offers the unique opportunity to gain power and energy.
How to observe/practice
Masi Magham is an appropriate day to destroy the ego and surrender oneself at the feet of the divine. Masi Magham is the ideal day to do Sathyanarayan puja at homes. Many Hindu devotees observe a full day fast on Masi Magham day and offer prayers to Lord Vishnu