The importance of Vaisakha and the mode of observance of the vrata are detailed in Skanda Purana.
The main features of the vow: early morning ablutions when the waters are believed to be sanctified by the presence of the Ganga and other gods and goddesses, feeding the poor, abstinence from stale food; partaking of food after it has been offered to the Lord, practising namasankirtana; listening to religious discourses; in short, participating in everything divine.
The third day in the bright half of Vaisakha, known as Akshaya Tritiya is the most auspicious day of the month - it is also the birth day of Balarama. This day is considered so holy that auspicious functions are held and charities are performed.
It is stated that “Lord Vishnu caused the Divine Ganga to flow on the earth on Vaisakha Sukla Tritiya.” It naturally follows that bathing in the Ganga and worshipping Lord Vishnu on the day are particularly propitious.
There was a king called Dharmikan in ancient Panchala. He was valorous and mighty and was endowed with prosperity. But, despite his valour and prowess, he lost his kingdom to his enemies and was exiled to the forest. There he lived the life of a hunter. Once he invoked his family guru to ascertain the cause of his suffering.
The guru told him that he had been a hunter in a previous birth. Never devoted to God, he indulged in such despicable acts like drinking, hunting, dacoity etc. Once he robbed two merchants and killed one of them. A maharshi who was watching this took fright and ran for his life. While doing so he fell down on the way on account of exhaustion. The hunter, however, saw him and tended him. By virtue of this act he was born as a king in his next life.
On the advice of the Guru, Dharmikan observed Vaisakha vrata in atonement. The Lord, pleased with his devotion appeared before him and blessed him. That day is Akshaya
Tritiya which is hence specially holy during the Vaisakha month.
While the observance of Vaisakha vrata is efficacious every where, at Guruvayur it is particularly so. A prince belonging to the ruling family of the erstwhile Travancore State was a great devotee. A saint presented him with a salagrama which the prince treasured and worshipped with great devotion. He and his subjects, however, suffered a series of disasters.
The prince sought the saint and asked his advice. He was told that if the salagrama was worshipped in the presence of Lord Vishnu good results would follow. The prince, therefore, resorted to the family deity Sri Padmanabha. Yet things did not improve.
Finally, be betook himself to Guruvayur and dedicated the salagrama to Guruvayurappan. Since then the princely family used to offer palpayasam during the midday puja to Guruvayurappan, which continued for several years.
These two legends indicate why Vaisakha is specially holy at Guruvayur and attracts hundreds of thousands of devotees. The devotees have darshan of the Lord during all the pujas; and participating in the sheevelis, listening to Bhagavata and other religious discourses, they remain in constant bliss. Four Bhagavata saptahas are arranged by the Devaswom to cover the entire Vaisakha month.