Vyāsa is the personality who delineated the glories of the names, form, qualities and pastimes of Bhagavān in this world. The ceremony to honour the ācārya who, sitting on a throne in the service of Bhagavān, preaches His glories and attracts people towards Him is called Vyāsa-pūjā. Another name for Vyāsa-pūjā is guru-pūjā. In India, the general convention is that guru-pūjā is observed on the day of Guru-pūrṇimā. It is considered Vyāsa’s appearance day, and on that day all sampradāyas worship their respective gurus. But in accordance with the scriptures, the primary ācārya of modern times, Śrī Śrīmad Bhaktisiddhānta Sarasvatī Gosvāmī Prabhupāda, established special worship of the guru on that guru’s own specific appearance day. Real guru-pūjā is when, on his appearance day, the guru worships his whole guru-paramparā and instructs his disciples on how to do the same.
In the scriptures, the glories of the guru have been described extensively. Śrīla Viśvanātha Cakravartī Ṭhākura has written:

yasya prasādād bhagavat-prasādo
yasyāprasādān na gatiḥ kuto ’pi

Śrī Gurvaṣṭaka(8)

By the mercy of the guru, one receives the mercy of Bhagavān. And if one doesn’t receive the mercy of the guru, he will never attain the kṛpā of Bhagavān. In Śrīmad Bhāgavatam(11.17.27) Śrī Kṛṣṇa says:

ācāryaṁ māṁ vijānīyān
nāvamanyeta karhicit
na martya-buddhyāsūyeta
sarva-deva-mayo guruḥ

One should know the guru as the āśraya-vigraha and nondifferent from Me. One should never disrespect him or attribute faults to him by perceiving him with material vision, for he is the embodiment of all the demigods.
There are numerous demigods and goddesses, and amongst them Brahmā, Viṣṇu and Maheśa are primary. The guru is the embodiment of Brahmā, the embodiment of Viṣṇu, and also the embodiment of Maheśa. He is compared to Brahmā because just as Brahmā creates this world, the guru creates bhakti by sowing the seed of devotion in our hearts. Viṣṇu is the maintainer, and gurudeva is he who maintains our bhakti. As long as we haven’t attained the stage of prema, he continues to strengthen our devotion. As conditioned souls, we cannot even imagine how much endeavour he makes for even one disciple. And as Maheśa is the destroyer, the guru destroys all of our anarthas and aparādhas. This is why the guru is said to be the embodiment of all the demigods.
There are so many jīvas in this world, and although some are inclined towards Bhagavān, most are averse to Him. Their intrinsic forms are as eternal servants of Kṛṣṇa, but forgetting this, they are wandering in material existence.
Without bhakti they will never possess any auspiciousness. Therefore sometimes Kṛṣṇa Himself descends into this world, sometimes He assumes different incarnations, and sometimes He sends His śakti in the form of the guru. Otherwise it would be impossible for the jīvas to ever receive auspiciousness. Only by bhakti can they attain their ultimate good fortune; yet bhakti is not a thing of this world. Inside the eternally perfected associates of Kṛṣṇa, the essence of the saṁvit-and hlādinī-śaktis is always present in the form of prema-bhakti. Until the jīva receives that essence, he will not possess any real auspiciousness. The guru is a resident of the spiritual world, and he descends into this world. He brings the prema of Goloka Vraja to this world and bestows it upon the conditioned souls. Such a great personality, an eternally perfected rāgātmikā devotee who possesses vraja-prema and brings it to this world, is known as a śuddha-guru.
As the current of the Gaṅgā carries water from the Himālayas down to the ocean, there is a current flowing through our guru-paramparā which, beginning from Kṛṣṇa Himself, has come down to the modern ācāryas and is presently flooding the entire material world with kṛṣṇa-prema. This is the primary function of the guru. If someone is not able to give this prema, then he is not really a guru at all. Being capable of bestowing kṛṣṇa-prema is the primary attribute of a genuine Vaiṣṇava guru.

—Śrīla Bhaktivedānta Nārāyaṇa Mahārāja (Śrī Prabandhāvalī)