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Long before the election of President Obama, Souter had expressed a desire to leave Washington, D.C., and return to his native New Hampshire.[27][28] The election of a Democratic president in 2008 made Souter more inclined to retire, but he did not want to create a situation in which there would be multiple vacancies at once.[27] Souter apparently became satisfied that no other justices planned to retire at the end of the Supreme Court's term in June 2009.[27] As a result, in mid-April 2009 he privately notified the White House of his intent to retire from the Supreme Court at the conclusion of its business for that term.[29] Souter sent a retirement letter to President Obama on May 1, effective at the start of the Supreme Court's 2009 summer recess.[30] Later that day President Obama made an unscheduled appearance during the daily White House press briefing to publicly announce Souter's retirement.[31] On May 26, 2009, Obama announced his selection of federal appeals court judge Sonia Sotomayor as the nominee. She was confirmed by the U.S. Senate on August 6.

On June 29, 2009, the last day of the Court's 2008–2009 term, Chief Justice Roberts read a letter to Souter that had been signed by all eight of his colleagues as well as retired Justice Sandra Day O'Connor, thanking him for his service, and Souter read a letter to his colleagues reciprocating their good wishes.[32]
Source, Wiki-pedia