Michelle Obama will talk about the president as a husband, father and friend, as well as the values that motivate his decisions, the officials said.
Obama told supporters in Virginia he'd be watching his wife's speech at home with the couple's two daughters.
"I am going to try not to let them see their daddy cry," he said. "Because when Michelle starts talking, I start getting all misty."
Castro, a Stanford University and Harvard Law School graduate who is the first Hispanic to deliver a convention keynote speech, will talk about his personal story and the future of the Democratic Party.
Hispanic Vote
Hispanics may account for 8.9 percent of the U.S. electorate in November, up from 7.4 percent in 2008, according to a report last month by the Center for Immigration Studies, a Washington-based research institute.
Support from Hispanics helped Obama win the White House four years ago, and this year their votes could be crucial in battleground states including Florida, Colorado and Nevada.
Also speaking tonight is Massachusetts Governor Deval Patrick, who will contrast Romney's vision of his achievements as that state's chief executive from 2003-2007 and what Democrats see as the negative impact of his economic policies.
Romney today is in Vermont preparing for October's three presidential debates and has no public events.
The convention's final night is scheduled to be moved from an indoor arena to the Charlotte's Bank of America outdoor sports stadium for Obama's speech. Convention planners say contingency plans call for holding the gathering in the smaller arena if severe thunderstorms move into the area.