Answer by taking a quote from the following article:

John Winston Ono Lennon  (9 October 1940 - 8 December 1980) was an English singer, songwriter, and peace activist who co-founded the Beatles, the most commercially successful band in the history of popular music. He and fellow member Paul McCartney formed a much-celebrated songwriting partnership. Along with George Harrison and Ringo Starr, the group would ascend to world-wide fame during the 1960s.

Lennon met Cynthia Powell (1939-2015) in 1957 when they were fellow students at the Liverpool College of Art. Although Powell was scared of Lennon's attitude and appearance, she heard that he was obsessed with the French actress Brigitte Bardot, so she dyed her hair blonde. Lennon asked her out, but when she said that she was engaged, he screamed out, "I didn't ask you to fuckin' marry me, did I?" She often accompanied him to Quarrymen gigs and travelled to Hamburg with McCartney's girlfriend to visit him. Lennon was jealous by nature and eventually grew possessive and often terrified Powell with his anger and physical violence. Lennon later said that until he met Ono, he had never questioned his chauvinistic attitude toward women. He said that the Beatles song "Getting Better" told his own story, "I used to be cruel to my woman, and physically--any woman. I was a hitter. I couldn't express myself and I hit. I fought men and I hit women. That is why I am always on about peace."  Recalling his July 1962 reaction when he learned that Cynthia was pregnant, Lennon said, "There's only one thing for it Cyn. We'll have to get married." The couple wed on 23 August at the Mount Pleasant Register Office in Liverpool, with Brian Epstein serving as best man. His marriage began just as Beatlemania was taking off across the UK. He performed on the evening of his wedding day and would continue to do so almost daily from then on. Epstein feared that fans would be alienated by the idea of a married Beatle, and he asked the Lennons to keep their marriage secret. Julian was born on 8 April 1963; Lennon was on tour at the time and did not see his infant son until three days later.  Cynthia attributed the start of the marriage breakdown to Lennon's use of LSD, and she felt that he slowly lost interest in her as a result of his use of the drug. When the group travelled by train to Bangor, Wales in 1967 for the Maharishi Yogi's Transcendental Meditation seminar, a policeman did not recognise her and stopped her from boarding. She later recalled how the incident seemed to symbolise the end of their marriage. After Cynthia arrived home at Kenwood, she found Lennon with Ono and left the house to stay with friends. Alexis Mardas later claimed to have slept with her that night, and a few weeks later he informed her that Lennon was seeking a divorce and custody of Julian on the grounds of her adultery with him. After negotiations, Lennon capitulated and agreed to let her divorce him on the same grounds. The case was settled out of court in November 1968, with Lennon giving her PS100,000 ($240,000 in US dollars at the time), a small annual payment and custody of Julian.

When did he meet Cynthia Lennon?
Lennon met Cynthia Powell (1939-2015) in 1957