What is Guru Purnima? What is the importance of Guru Purnima? Is it essential to follow (observe) this (festival) day?

Today we will listen to something about Sri Guru Tattva. What is Guru Purnima? What is the importance of Guru Purnima? Is it essential to follow (observe) this (festival) day? 

Guru Purnima is not the birthday of Sri Gurudev but you can say it is the installation day of Guru Tattva. It is said that on this day, Svayam Bhagavan, gave mantra to Brahma and that was the first initiation. Bhagavan inaugurated Guru Tattva, guru varga (disciplic line, parampara) for the jivas of this world and from Brahma it is coming down. Initiation, receiving mantra and receiving the power of the mantra, originates from Bhagavan (and descends through Brahma to Narada to Vyasadev to later Vaisnava Acaryas and to the present day Guru through an unbroken succession of genuine Gurus in a genuine tradition). That is why it is absolutely essential to follow such a tradition. Otherwise, (the mantra will be devoid of potency). Mantras are written in the book. You can find all the mantras in different scriptures, but you must receive the power of the mantra from Sri Guru.  And what is the importance of getting it from Sri Gurudev? Sri Gurudev will infuse us with that power, the power of the mantra. Because the mantra he gives is invested with that power (potency).  Before the creation, after Brahma observed prarabhda kala (?), tapasya (severe austerities), Bhagavan gave him initiation and said, “Now, just do your sadhana and meditate on this mantra and by this mantra you will be able to manifest everything, the whole of material creation. The seed of creation is already there in the mantra. I have already put all the seeds in the mantra and you have to manifest the creation out of this seed mantra.” 

So there is power in this mantra, the power of creativity. There is power to develop our eternal and very intimate connection, direct to the Supreme Godhead, Krishna. So the power comes from Bhagavan and is carried down through all the Gurus in the tradition. Sri Guru receives it from param-guru, who gets it from parameṣthi-guru, from paratparam-guru and so on (all the way up to Bhagavan). That is why it is absolutely essential to receive that mantra from within the tradition. Otherwise the mantra will be there but that power which comes down from Sri Krishna Bhagavan Himself will not be there (The mantra will be devoid of potency, like a blank bullet).