Judge Tony Baumgartner, the recorder of Westminster, said: “This was well-planned offending and you knew precisely what you were doing.
“Foolishly you climbed a far way up the tower, far enough to cause very serious harm to you and possibly others if you fell.”
The judge found Day had “little insight into your offending or any real remorse”, but he noted he had not offended since the Big Ben incident and had strong personal mitigation including failing eyesight.
Day was ordered to attend up to 20 rehabilitation days and he will be electronically monitored for four months as part of his suspended sentence.
Prosecutor David Matthew previously told the court that Day’s actions triggered an emergency services response which caused “serious disruption”, including the closure of Westminster Bridge.
Jurors were told that mattresses were put down at the base of the tower to try to protect him because he was in a “precarious position”.
Transport for London (TfL) lost an estimated £25,000 in bus fares as diversions were put in place and routes curtailed, jurors were told.
CCTV played in court showed Day, wearing a black coat and a baseball cap, scaling the building barefoot.