There are different opinions regarding Sri Visvarupa Mahotsava. It's the day when Sri Visvarupa (elder brother of Sri Caitanya Mahaprabhu) accepted sannyasa at the age of 16 from Sripada Krsna Bharati in Kasi (Varanasi) in 1491 and accepted the name Sri Sankararanya (Sri Sri Gaudiya Vaisnava Abhidana & Acarya Folio), OR, it's the day he departed from this world in Pandharpur on the banks of the Bhima River by entering the Deity of Sri Viththalanatha (CB 7.73 - commentary by Bhaktisiddanta Maharaja) in the year 1509/1510 (there are two opinions: either 1431 Sakabda Era-A.D. 1509, or, 1432 Sakabda-A.D. 1510), OR, both pastimes occurred on the same day, different years. Afterwards, while Mahaprabhu was traveling in South India in 1511, He heard from Sri Ranga-puri about Sri Visvarupa's disappearance.
It is also "believed" that Sri Caitanya Mahaprabhu also took sannyasa on this day and began to search for his brother, Sri Visvarupa, but we can not find any sastric references confirming this. According to Sri Caitanya-caritamrta (Madhya 3.3): "At the end of His twenty-fourth year, in the month of Magha, Sri Caitanya Mahaprabhu accepted the sannyasa order during the waxing period of the moon." And in the Sri Caitanya-bhagavata (Madhya 28.9) “On the coming Makara-sankranti day, I will certainly leave home and take sannyasa." The year was 1510. Makara-sankranti is in the month of Magha (January/Febuary) when the sun enters into the direction of the North (the 6th/7th day of the Magha month). Sri Visvarupa-mahotsava falls on the Purnima (full moon) in the month of Bhadrapada (August/September).
Srila Bhaktisiddhanta Maharaja states in his commmentary (CB Adi 7.96) regarding who Sri Visvarupa is: "Sri Visvarupa Prabhu is Sankarsana and is therefore nondifferent from Sri Nityananda Svarupa. The prakasa expansion of Mula-Sankarsana Sri Baladeva- Nityananda Prabhu in Maha-Vaikuntha has appeared as Visvarupa in Gaura-lila. If one hears the pastime of Visvarupa’s accepting sannyasa, he will attain freedom from the bondage of fruitive activities. Sri Visvarupa has three expansions—the first purusa incarnation, Karanodakasayi Visnu, the second purusa incarnation, Garbhodakasayi Visnu, and the third purusa incarnation, Ksirodakasayi Visnu. If a living entity understands these three Visnu forms in truth, he can attain freedom from the material concept of life."
As cited by Srila Prabhupada in his purport (CC Adi 13.74) regarding Sri Visvarupa: "As an incarnation of Sankarsana, He is both the ingredient and immediate cause of the creation of this material world. He is nondifferent from Sri Caitanya Mahaprabhu, for the amsa and the amsi, or the part and the whole, are not different. As an incarnation of Sankarsana, Visvarupa belongs to the quadruple manifestation of catur-vyuha. In the Gaura-candrodaya it is said that Visvarupa, after His so-called demise, remained mixed within Sri Nityananda Prabhu."
According to HH Narayana Maharaja, there is another explanation who Sri Visvarupa was: "Who, then, was Visvarupa? He was one of the manifestations of Baladeva Prabhu. He was not Nityananda. He was a manifestation of Nityananda. He was like Maha-Sankarsana. He left Navadvipa and went to Pandarapura. When He disappeared from this world he merged in Nityananda Prabhu, just as Baladeva Prabhu who was the son of Devaki merged in Rohininandan Rama (after being in Devaki's womb for six months). It was in this way. Visvarupa was not fortunate enough to be with Mahaprabhu in performing all His lilas, and He was therefore not Nityananda Prabhu-he was like Maha-Sankarsana. So we can reconcile it like this."
And of course Sri Visvarupa Mahotsava is also the day when our dear Srila Prabhupada accepted sannyasa from Srila Bhakti prajna Kesava Maharaja at the Sri Kesavaji Gaudiya Matha, in Mathura, India, in 1959.(courtesy of Jaya Vijaya dasa. 11th September, 2011.)
It is said that Çaìkaräraëya
was the sannyäsa name of Çréla Viçvarüpa,
who was the elder brother of Viçvambhara (the original name of Çré
Caitanya Mahäprabhu). Çaìkaräraëya expired
in 1432 Çakäbda (A.D. 1510) at Sholapur, where there is a place
of pilgrimage known as Päëòarapura. This is referred to
in the Madhya-lélä, Chapter Nine, verses 299 and 300.
(Srila A.C. Bhaktivedanta Swami
Prabhupada. Sri Chaitanya Charitamrita Adi-lila 10:106 purport.)
Viçvarüpa was the elder
brother of Gaurahari, Lord Çré Caitanya Mahäprabhu.
When arrangements were being made for the marriage of Viçvarüpa,
He took sannyäsa and left home. He took the sannyäsa name of
Çaìkaräraëya. In 1431 Çakäbda Era (A.D.
1509), He disappeared in Päëòarapura, in the district
of Sholapur. As an incarnation of Saìkarñaëa, He is
both the ingredient and immediate cause of the creation of this material
world. He is nondifferent from Çré Caitanya Mahäprabhu,
for the aàça and the aàçé, or the part
and the whole, are not different. As an incarnation of Saìkarñaëa,
Viçvarüpa belongs to the quadruple manifestation of catur-vyüha.
In the Gaura-candrodaya it is said that Viçvarüpa, after His
so-called demise, remained mixed within Çré Nityänanda
Prabhu.
(Srila A.C. Bhaktivedanta Swami
Prabhupada. Sri Chaitanya Charitamrita Adi-lila 13:74 purport.)
ataeva prabhu täìre bale,
‘baòa bhäi’
kåñëa, balaräma
dui——caitanya, nitäi
TRANSLATION
Because Mahä-saìkarñaëa
is the ingredient and efficient cause of the cosmic manifestation, He is
present in every detail of it. Lord Caitanya therefore called Him His elder
brother. The two brothers are known as Kåñëa and Balaräma
in the spiritual world, but at the present moment they are Caitanya and
Nitäi. Therefore the conclusion is that Nityänanda Prabhu is
the original Saìkarñaëa, Baladeva.
(Srila A.C. Bhaktivedanta Swami
Prabhupada. Sri Chaitanya Charitamrita Adi-lila 13:78 text and translation.)
Read Sri Chaitanya Charitamrita Madhya lila 7th chapter.
sannyäsa kari’ viçvarüpa
giyäche dakñiëe
avaçya kariba ämi täìra
anveñaëe
TRANSLATION
“My elder brother, Viçvarüpa,
has taken sannyäsa and gone to South India. Now I must go search for
Him." 44
äjïä deha, avaçya
ämi dakñiëe caliba
tomära äjïäte
sukhe leuöi’ äsiba’
TRANSLATION
“Please permit Me to go, for I must
tour South India. With your permission, I shall soon return very happily.”
45
çuni’ särvabhauma hailä
atyanta kätara
caraëe dhariyä kahe viñäda-uttara
TRANSLATION
Upon hearing this, Särvabhauma
Bhaööäcärya became very much agitated. Catching hold
of the lotus feet of Caitanya Mahäprabhu, he gave this sorrowful reply.
46
bhaööäcärya-saìge
ära yata nija-gaëa
jagannätha pradakñiëa
kari’ karilä gamana
TRANSLATION
Accompanied by His personal associates
and Särvabhauma Bhaööäcärya, Çré
Caitanya Mahäprabhu circumambulated the altar of Jagannätha.
The Lord then departed on His South Indian tour. 58
http://www.prabhupada.com/rare-callender.html