Washington (CNN) -- President Barack Obama officially begins his second term today, a day ahead of his public swearing in and all the pomp and ceremony of a U.S. presidential inauguration.
Because the Constitution calls for the president to be sworn in on January 20, Obama and Vice President Joe Biden will be sworn in for their second terms in relatively quiet ceremonies before they take their public oaths in front of the Capitol on Monday and before an audience spread down the National Mall.
Obama's Monday address will lay out the values and vision for his second term and acknowledge the division in Washington but won't address policy, a source with knowledge of the speech told CNN on the condition of anonymity. He'll lay out policy in his February 12 State of the Union address, the source said.
Obama was still working on his speech on Saturday, the source said.
The nation's first African-American president also will become only the 17th U.S. leader to deliver a second inaugural address before joining the traditional parade up Pennsylvania Avenue to the White House.
For his second inauguration, less than half of the estimated 1.8 million onlookers who crammed the Mall in 2009 are expected -- organizers expect 800,000 people to attend Monday's public ceremony.
The smaller crowd this time around reflects the reality of second-term presidencies, when the novelty and expectations of a new leader have been replaced with the familiarity and experiences of the first four years.
Inauguration activities kicked off on Saturday with Obama and first lady Michelle Obama and Biden and his wife Dr. Jill Biden leading volunteers across the country in National Day of Service Activities.
The Obamas joined in a project at Burrville Elementary School in Washington, aiding volunteers who were staining a bookcase. Cameras at the school caught the president and first lady staining a bookcase.
The president told volunteers that his family would do volunteer projects on holidays, "So I was taught from a young age." Volunteering "is really what America is all about," he said.
The Bidens helped to put together care packages for service members deployed overseas at the National Guard Armory in Washington. Biden's office said volunteers at the armory would produce 100,000 packages.
"We still have 68,000 troops in harm's way in some of the most godforsaken territory in the world," Biden said, adding that the military members can find comfort "knowing that we back home just remember, we know what's going on."
Chelsea Clinton, honorary chairwoman of the Day of Service, said at a kickoff event on Mall that Saturday was the 19th anniversary of her father former President Bill Clinton signing the bill that designated a National Day of Service to coincide with Martin Luther King Jr. Day, the federal holiday honoring the late civil rights leader.
"When he signed the bill, he reminded us of what Dr. King called life's most persistent and urgent question: What are you doing for others?" she said. "And in my family, the only wrong answer to that question is 'nothing.' "
Later Saturday, singer Katy Perry headlined a concert for children of service men and women and Washington schoolchildren that was hosted by the first lady and Mrs. Biden. Singer Usher and the cast of the TV show "Glee" were among others who performed.
The Saturday event was to recognize the sacrifices and "level of maturity that is required from military kids," the first lady said.
"It means always thinking about things that are so much bigger than yourself. It means growing up just a little faster and working just a little harder than other kids," she said. "And it means doing the greatest thing you can ever do with your life at such a young age, and that is to serve our country."
On Sunday, the vice president will take his official oath shortly after 8 a.m. at the Naval Observatory, his official residence, and the president will take his at the White House shortly before noon.
Obama and Biden will travel to Arlington National Cemetery after Biden's swearing-in for a traditional wreath-laying ceremony.
In the evening, the Obamas will watch Latino acts at "In Performance at the Kennedy Center," which is followed by the Let Freedom Ring concert. The Red, White and Blue Inaugural Ball and Hip-Hop Inaugural Ball are also scheduled in the capital.
The president will speak to donors at a Candlelight Celebration and the National Building Museum on Sunday night.
Monday's events will be a bit down-sized from Obama's first inauguration. After events in front of the Capitol, the Obamas and Bidens will lead the traditional parade down Pennsylvania Avenue to the White House, there are only two presidential balls this year, down from the 10 staged in 2009.
While the anticipated crowd for Monday's events is expected to be about half of those who gathered four years ago, the temperature will be a bit higher than in 2009 when the high hovered around the freezing mark. While the early morning temperature will be in the 20s, the forecast calls for a high temperature in the upper 30s or low 40s. Still, organizers cautioned attendees to bundle up because of prolonged exposure to the cold as they watch the events and make their way to and from them.