As America continues to grieve over the tragedy in Tucson that killed six and injured 19, the husband of Arizona congresswoman Gabrielle Giffords is issuing a call to action.

Captain Mark Kelly, Navy pilot and NASA astronaut, is asking communities to honor Dr. Martin Luther King, Jr. Day by dedicating a few hours of service on the holiday. Kelly posted a letter on his wife's website thanking the public for all the prayers and good wishes his family has received and encouraged these communities to continue their powerful outpouring of support.

Kelly also asked people to celebrate the legacy of Dr. Martin Luther King, Jr. "What united the victims of the tragedy on Saturday was service - they volunteered in church or at soup kitchens, worked in government, and tended to their communities," Kelly writes. "On behalf of Gabby and our family, I ask that you consider honoring their commitment to service by dedicating a few hours on Dr. Martin Luther King, Jr. Day."

Rep. Giffords continues to make steady progress in her recovery. Doctors removed her breathing tube Saturday, freeing the congresswoman from the ventilator and allowing her to breathe unassisted. Giffords is opening her eyes and moving limbs on both sides of her body.

U.S. Attorney General Eric Holder delivered another message of service in Atlanta Sunday. He spoke at the Ebenezer Baptist Church in observance of the holiday.

Holder reminded the congregation that 40 years after King's assassination, the long struggle to end violence and promote peace continues.

"In times of heartbreak and bereavement, the power of Dr. King's example becomes even clearer," Holder said.

"Let us recommit ourselves to carrying on Dr. King's work and to honoring the values that were at the center of his life: tolerance, non-violence, compassion, love and, above all, justice."