At the heart of the weekend is the Sport Relief mile, where many thousands of people will run a mile for charity.
Celebrities like David Beckham are urging people to join in, whatever their age.
The 16 main events will be held in Belfast, Birmingham, Brighton, Bristol, Cardiff, Gateshead, Glasgow, Hull, Leeds, London, Manchester, Milton Keynes, Norwich, Nottingham, Plymouth and Southampton, but there are hundreds of other smaller ones too.
James Corden trains Wayne Rooney
Sport Relief star David Walliams has been getting on his bike for Sport Relief and has already battled through temperatures of -15C through the highlands of Scotland as he starts his challenge to cycle the length of the UK.
And in the recording studio Robbie Williams has made a Sport Relief single called Morning Sun.
In another of James Corden's contributions to the charity weekend, he gives Manchester United a training session and takes them through a gruelling regime.
Blue Peter's Helen Skelton has taken six weeks to travel the length of the Amazon River in a kayak!
It's hoped she has broken the Guinness World Record for the longest solo journey by kayak and has become the first woman in history to solo kayak the Amazon.