Bitcoin reaches $1.00 USD for the first time on the Mt. Gox exchange on February 9, 2011. From 1,309 BTC per dollar in October 2009 to parity in under two years. The psychological significance was enormous -- one bitcoin was now worth one dollar, a benchmark that made the abstract concept of digital money tangible. By June 2011, the price would surge to over $31, but this was the moment Bitcoin first proved it could hold real monetary value.
Wikipedia: History of Bitcoin