Here’s a really good explanation found on the High Asia Tours website.
Nepali New Year, the first day of the Baisakh Month, which usually falls mid April is celebrated all over Nepal but the celebration at Bhaktapur is special due to its connection with Bisket Jatra Festival. If you are traveling to Nepal during April, it's worth attending the celebration in Bhaktapur. Bisket Jatra means the Celebration after the death of serpents.
<br class="kix-line-break" />The legend goes that every man who married the Bhaktapur Princess died the night of honeymoon and so no one dared to marry the Bhaktapur Princess again. Finally, there was a brave prince who vowed to solved the mystery. He married the princess and he stayed awake the night of honeymoon. He saw that as the princess fell asleep, two giant serpents crawled out of the two nostrils of the princess. The prince quickly took out his sword and chopped the snake heads off. The next day morning, the two serpents were publicly displayed in a pole. Till today, Bisket Jatra carries the tradition of the displaying the serpents in the form of long ribbons.
The festival begins 4 days before the Nepali New Year Day and lasts total 9 days. The festival has 5 major parts; pulling of the chariot temple of Lord Bhairava and Goddess Bhadrakali, erecting the Yeo-sin-deo pole, lowering of the pole, chariot temple processions of Goddess Mahalaxmi and Lord Mahakala, Brahmayini and Lord Ganesha and the rituals to revere all the Hindu deities attending the festival.
<br class="kix-line-break" />The pulling of the chariot temples of Lord Bhairava and Goddess Bhadrakali, the male and female manifestation of Lord Shiva and his consort Parvati respectively, begins 4 days before the New Year Eve Day. Caretaker priests of the respective temples perform pre-outing rituals and receive the state offerings for the deities. After the rituals and worships, the priests carry the images of the deities to their respective chariot-temples and hold the images on their arms and be seated in the inner sanctum of the chariot. Another priest holding the sword from Bhaktapur's Palace guards the image.
<br class="kix-line-break" />Then the residents of Bhaktapur start pulling of the chariot temples for the nine-day festival trip. The creaking and swaying chariot lumbers around town, pausing for a huge tug of war between the eastern and western sides of town. After the battle the chariots head to Khalna Tole. Every evening, the chariots are parked in certain areas and the images of the Lord and Goddess are taken to the temporary residences. There will be lots of worships and even animal sacrifices to appease the God on the first and the third day.
<br class="kix-line-break" />The chariot ride continues on the fourth day to Khalna Tole-the site of erecting of pole called Yeo-sin deo .It's a huge 80 ft. lingam (phallic symbol) is erected in the stone yoni (female genital symbol) base. The preparation of erecting of the pole begins around afternoon. Priests perform tantric worships that involves a male buffalo sacrifice. A tree trunk is tied about few feet below the top end of the pole to form a cross-which actually are the arms of the "Yeo-sin-deo". The pole is decorated with the face mask of Lord Akash Bhairav and bunch of twigs and two long ribbons or flags representing two serpents. There will be rituals and worships and with the help of bamboo and the ropes, people erect the pole with huge applause and cheers. <br class="kix-line-break" />After the pole stands erect, the deities are carried and enshrined by the pole for the night. Again there will be series of worships all night.
<br class="kix-line-break" />On the New Year day i.e. on the fifth day of the festival, people take ritual wash and bath in the waters of Hanumantay River and pay homage to the Yeo-sin-deo and Lord Bhairav and Goddess Bhadrakali. People believe that taking a purification dip in the water of the stream on this day, they keep all the diseases away for that year. Again, there will be rituals and animal sacrifices to the deities.
<br class="kix-line-break" />Towards the end of the day, the Yeo-sin-deo pole is pulled down amidst a huge cheering crowd and people run over each other to get a piece of the cotton tied in the pole. After some tantric offerings, the touring deities ride to their temporary residence in the designated places. The chariot tour of the deities continues for four more days and on the last day, the deities are back to their own temples and state swords are returned back to the palaces.
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