Describe The Poem The Eve Of St. Agnes By Keats And With What Subject Deals This Poem?

In this poem, Keats sees an ideal beloved and an embodiment of true and sincere love in the shape of Medeline. The poem deals with the well-worn subject of love between a brave youth and a lady of hostile clan. The story is based upon the popular superstition connected with the eve of st. Agnes. It was a popular belief that if a girl performed certain ceremonies on this occasion, she might dream of the person whom she was destined to marry. Once on the eve of st. Agnes a feast was arranged in the castle of a baron.

Medeline, the daughter of the baron, wanted to have a sight of her future husband. She performed the proper rites and went to her bed. Outside the castle it was intense cold. In the great hall, guests were busy in merry-making.Prophyro, a young man of enemy tribe, loved Medeline from the core of his heart. He wanted to see his beloved. So, at the risk of his life, he reached the castle. Medeline's old nurse, Angela, helped him and he obtained access to her room. After a short while Medeline came there. She undressed herself and slipped into her bed.

Prophyro came out of the corner and began to sing a song.  Medeline was awakened from her sound sleep. She looked at Prophyro and took him for a vision. But Prophyro explained the whole situation and requested her to run away with him. She agreed and they started towards the gate of the castle. Her father and the guests were fast asleep. So, none could notice them leaving the castle. The baron and his guests had strange and dreadful dreams that night. The old nurse, Angela, also died.

St Agnes was a Roman virgin and martyr during the reign of Diocletian (early 4th century.)  At first condemned to debauchery in a public brothel before her execution, her virginity was preserved by thunder and lightning from Heaven.  Eight days after her execution, her parents visited her tomb and were greeted by a chorus of angels, including Agnes herself, with a white lamb at her side.
The Eve of St Agnes was written at Chichester and Bedhampton during the last half of January 1819.  Perhaps Keats was inspired by the calendar - St Agnes's feast is celebrated on 21 January.  He revised the work at Winchester in September; it was first published in 1820.  
On the eve of St Agnes's feast day (20 January), virgins used divinations to 'discover' their future husbands.  As Keats writes:  '[U]pon St Agnes' Eve, / Young virgins might have visions of delight, / And soft adorings from their loves receive'.  
The poem tells the story of Madeline and her lover Porphyro.  It is one of Keats's best-loved works.  It also inspired numerous pre-Raphaelite paintings.
Keats based his poem on the superstition that a girl could see her future husband in a dream if she performed certain rites on the eve of St. Agnes; if she went to bed without looking behind her and lay on her back with her hands under her head, he would appear in her dream, kiss her, and feast with her.
In the original version of this poem, Keats emphasized the young lovers' sexuality, but his publishers, who feared public reaction, forced him to tone down the eroticism.