Touchdown at the Kennedy Space Center in Florida has been initially targeted for 9:15 a.m. EDT (1315 GMT) but bad weather over the region could delay this.
The mission had been slated to end Friday, but clouds and rain showers over Florida prompted flight directors to keep the crew in orbit an extra day to see if the weather would clear.
"The weather is still marginal," astronaut Greg Johnson from the Mission Control Center in Houston, told the crew on Saturday.
NASA will decide by 8 a.m. whether to go for their first touchdown opportunity in Florida. Meteorologists late on Friday were predicting continued cloudiness and a chance of rain, either of which would violate the space agency's criteria for a safe landing.
NASA also is staffing its backup landing site at Edwards Air Force Base in California's Mojave Desert. The first landing opportunity there would occur at 10:44 a.m. EDT (1444 GMT).
Flight directors could decide to keep the shuttle in orbit another day if Florida's weather forecast for Sunday is better.
Landing in California costs NASA more than $1 million and it takes a week or two to prepare and transport the shuttle on top of a jet carrier aircraft back to Florida for its next launch.
Atlantis has enough supplies to stay in orbit until Monday.
Hubble Telescope Upgraded
The agency is trying to finish up eight flights on the shuttle's manifest before the shuttle fleet is retired by the end of 2010. NASA is targeting its next shuttle mission for launch in three weeks.
Shuttle Endeavour, which was on standby as a rescue vehicle for the Atlantis crew, is scheduled to deliver an outdoor porch for the Japanese laboratory on the International Space Station.
NASA wanted the standby rescue capability since the Atlantis crew on their mission to the Hubble observatory could not reach the station if their ship was too damaged for the supersonic return flight through the atmosphere for landing.
NASA has been worried about possible heat shield damage to shuttles since the 2003 loss of shuttle Columbia.
The Atlantis crew had used laser imagers to inspect their ship's heat shield after reaching orbit. NASA determined the shuttle is in good shape for landing.
On their mission, the astronauts conducted five consecutive days of spacewalks to refurbish the highly productive Hubble telescope, which has been in orbit since 1990.
Hubble was outfitted with a new panchromatic wide-field camera that is expected to be able to image objects formed about 500 million years after the birth of the universe.
The Atlantis astronauts also left Hubble with a new light-splitting spectrograph to investigate the chemical composition of gas and dust between the galaxies.
Astronomers are curious to learn just how early structures like galaxies came into existence.
The first images from the rejuvenated Hubble are expected in September.
