Magha Purnima also known as Maghi Purnima is a full moon day which falls in the month of Magh (11th month in Hindu calendar.) Magha is one of the most sacred months because at the beginning of the month the sun sets on its northern path. Magha Purnima is the last and most important day of the month of Magh. It is regarded as an auspicious day to take holy bath.
Magha Purnima also marks the conclusion of the Magha month in North India. Therefore, the full moon day of this month is greatly revered by Hindu devotees especially the worshipers Lord Vishnu.
Magha Mela which is an annual event is held at Prayag in Allahabad and millions of devotees including pilgrims, underprivileged, priests, ascetics and the common man take a holy bath on this day.
Magh Purnima is venerated as a 'bathing festival' across the country and a dip in the Ganga on this day holds high religious merit. Hindu devotees take a holy bath in the Ganga or Yamuna rivers every day of the month which begins from Paush Purnima and ends on Magha Purnima. Those who cannot make it to the sacred river can take a bath in any other stream, river, tank or pond which is regarded as holy and auspicious. People walk for miles and miles to be able to take a single holy dip in the river. On this day, a number of bathing festivals are held on the banks of holy rivers like the Ganga, Jamuna, Krishna, Tapti, Kaveri.
Lord Vishnu is worshiped on this day and just like all other purnima days, Satyanarayana Vratham is observed. Lord Vishnu is offered kumkum, til, fruits, supari, leaves of banana, pan and moli. Panchamrit is made out of ganga jal, honey, milk, tulsi and sweets. Prasad is prepared mixing wheat and sugar together and roasting them. There are religious sermons held to celebrate the occasion at auspicious places. Lord Shiva, Lord Brahma and Goddess Laxmi are also worshiped.
A suggested mantra to worship lord Vishnu (the sustainer) during February full moon celebration can be Narayana Vidmahe (Narayana = Vishnu) It is mainly a morning mantra... a deep prayer.