The purchase of Alaska by the United States occurred in 1867 at the behest of Secretary of State William Seward.
Seward convinced the United States Senate to approve the purchase (about 600,000 square miles) from Russia for $7,200,000 on April 9th, 1867 (the treaty was signed on March 30). The Senate approved the purchase by just one vote. The treaty was negotiated with Baron Eduard de Stoeckl, Russian minister to the United States. For comparison, $7.2 million in 1867 is equivalent to about $86.7 million in 2002 dollars.
The purchase was at the time derided as Seward's folly, Seward's icebox and President Andrew Johnson's polar bear garden, because it was believed foolhardy to spend so much money on the remote outpost. However, few today would consider it to be a mistake. Alaska has proven to be abundant with natural resources such as petroleum, natural gas, gold, coal, timber, and salmon. However, an analysis of the cash flow from the investment compared with the net present value of its purchase price suggests the money could have been invested better elsewhere. 
Currently, Alaska celebrates the purchase on Seward's Day, the last Monday of March.