The Olympic Games are held every four years, each time in a different city. It is the city that hosts the games, not the country. Judo was admitted as an 'exhibition' sport for the Tokyo games in 1964. This means that the IOC is 'considering' how it works within the Olympic framework and have not yet committed to including it as a permanent fixture. In did not feature in 1968. Returning in 1972, women were finally allowed to compete in 1992, thereby showing that sexism was still rife. Information can be found on the Olympic Games website and here. A full list of medal winners up to 2004 is here.
First held in 1956 this contest is held every two years except that in 1961 it swapped to the odd years, presumably to avoid clashing with the Olympics. 1961 was also the first year that it was held outside of Japan and is famously also the first occasion that the Japanese were beaten (by Anton Geesink). It is the country that competes and not the individual, so, for example, when Neil Adams 'won' in 1981, it was Great Britain that won. Further information can be found on Wikipedia, the 2009 results are here with the rest of the results here.
This championship is unusual in that there are no weight categories. There are 'men' and 'women' and that is it. Yashuhiro Yamashita has won it the most with nine wins. A short article is on Wikipedia and the records of winners are here.
Red & White
The Red and White Championships are held twice a year in Tokyo. Initiated by Kano as the very first tournament it has unusual rules. The contestants are lined up in order of rank and within that, size/weight. The contest starts off at the bottom and the winner of the first contest goes on to fight the second contest and so on and on. The eventual winner of the championship is the person who has scored the most wins. However, there is another angle. If any contestant manages to win five fights (or more) they get an instant promotion. This is called a 'Batsagan'. You can download a Word document explaining in greater detail from here.
An international competition held as part of the IJF Grand Prix series. It is held 'roughly' every two years (depending upon what else is going on in the Judo world) at the Kodokan in Tokyo. The record of the games are kept here.
The Asian games are held every four years on the even years alternating with the Olympics although after the 2014 games the next will be held in 2019.
Judo has been part of these games since 1986. There is very little specific information on Judo within these games but a general article on the Asian games as a whole is on Wikipedia.
This championship is held every year (with a few exceptions) in various cities throughout Europe. There is more information available on the European Judo Union website. A full list of medal winners can be found on Wikipedia where you select a year and click on the 'details' and it takes you to the relevant page.
There are also a number of other international competitions such as the Tornoi de Paris, the Pan American and the Commonwealth Games.
Below are the main Tournaments within the BJC Calendar.
BJC Junior Nationals
This competition is normally held in the spring of each year. It is open to children from 8 years to the day before their 16th birthday. For those who are 16 and above but less than 21 there is an Espoirs category.
The competition is normally held in Hereford (as are most BJC competitions) and is well attended from all across Great Britain. The contestants are split into weight groups. There is not an open category.
BJC Senior Nationals
Also held in Hereford, the BJC Seniors are based in a similar manner to the Juniors. Although open to Judoka of any age above 16, it is not normal to see contestants in their 40's or older take part.
BJC Welsh Nationals
Also held in Hereford, for some reason, the Welsh Nationals is open to anyone who is a member of the BJC whether they live in Wales or not.
It is run on the same basis as the competitions above.
BJC Grade & Vets.
This competition is normally held in Leicester but does sometimes visit other towns such as High Wycombe in 2009.
There are two main categories. The 'grade' (meaning those below black belt) fight in weight groups if they are below 35 years of age. The 'vets' (meaning those older than 35 years) fight in open categories of Kyu mixed together or Dan grade. This is a Batsagan contest. Apart from gaining a medal, there is also the opportunity if one is successful enough, to be promoted one or more grades.
Many other championships are held on an area basis that are open to any BJC Judoka. The BJC website maintains an up to date list here.