Sky Ride is a nationwide campaign to get as many people in the UK as possible off their backsides and onto bikes. Events take place all around the country throughout the year, and on the 4th of September, it's London's turn.
In 2010, the mayor's London Sky Ride day attracted more than 80,000 cyclists to the centre of the capital, where they could enjoy a free and safe ride on streets usually packed with cars, lorries and buses.
This year, on the 4th of September, it's happening again. Mayor Boris is closing off a large number of streets in central London to regular traffic so that you can have a fun day out on two wheels.
Imagine cycling at a leisurely pace past some of London's most famous landmarks without being bothered by speeding vehicles and harmful traffic pollution.
Look out for various activities and entertainment along the route, as well as plenty of places to grab some refreshments.
Absolutely everyone is encouraged to go along and get involved, from toddlers with stabilisers, to senior citizens (possibly also with stabilisers). Of course, an event like this is perfect for a family day out.
Don't have a bike? That's no excuse. The Sky Ride's website has a list of places where you can hire bikes for a reasonable fee.
The route covers just under 10 miles (15km), though of course you don't have to cycle the whole of it. You can join it and leave it at any point - it's not a race. For details of the route, click here and then look for the "route" tab on the webpage.
The London Sky Ride is free to join, though the organisers would appreciate it if you took five minutes to register your interest online, just so that they can get an idea of the numbers that might turn up on the day.
Sky Ride's website suggests cyclists take along the following on the day: a helmet, snacks and liquids, sun cream, and a big smile.