NASA said Kelly will take part in a news conference from Johnson Space Center in Houston.
His identical twin, Scott, hinted in an interview this week from the International Space Station that his brother would choose to fly in April.
Kelly took a leave from training after Giffords was gunned down at a Tucson, Ariz., shopping center on Jan. 8. She's undergoing rehab in Houston.
Kelly spent the past month debating whether to step down as commander of Endeavour's two-week mission. Liftoff is targeted for April 19. It will be Endeavour's final flight.
