Churches and homes throughout the southern Arizona city rang bells at 10:11 a.m. MST, the exact time the gunman shot Giffords and methodically moved down a line of people waiting to meet her during a congressional meet-and-greet outside a supermarket on Jan. 8, 2011.
Saturday, Giffords' husband, former astronaut Mark Kelly posted a photo via Twitter, showing Giffords returning to the parking spot she chose on the day of the shooting.
The couple also visited University Medical Center, where Giffords was treated after the attack. She led the crowd in the pledge of allegiance.
More bells tolled later Sunday at Tucson's packed St. Augustine Cathedral, where hundreds of people gathered, including Gov. Jan Brewer.
The names of the six people killed in the shooting rampage were read. They stood and chanted, "We remember, we remember, we remember with grateful hearts."
Relatives of the six dead walked solemnly down the aisle with a single red rose, placing the flowers in a vase in front of a picture of a heart.
At Los Angeles City Hall, the bell rang six times in memory of the people shot and killed.
City Councilman Tom LaBonge says Tucson's mayor asked elected leaders in cities across the country to pay tribute to the victims.
Friday, the congresswoman participated in a ceremony honoring her late staff member, Gabe Zimmerman, who died in the rampage.
Among the killed were a 9-year-old girl born on Sept. 11 and a federal judge. Thirteen others were wounded, including Giffords.
The 41-year-old congresswoman has spent the last year in Houston undergoing intensive physical and speech therapy.
Doctors and family members have called her recovery miraculous after the shooting. She is able to walk and talk, vote in Congress and gave a televised interview to ABC's Diane Sawyer in May.
However, doctors have said it would take many more months to determine the lasting effects of her brain injury. The three-term congresswoman has four months to decide whether to seek re-election.
Giffords and Kelly are planning to join thousands at an evening candlelight vigil at the University of Arizona. Kelly was expected to speak.
Jared Lee Loughner, 23, has pleaded not guilty to 49 charges in the shooting. Loughner, who has been diagnosed with bipolar disorder, is being forcibly medicated at a Missouri prison facility in an effort to make him mentally ready for trial.
The Associated Press contributed to this report.