The IJSO is organised in late November or early December by the National Organising Committee (OC) of one of the participating countries on behalf of its National Government, its Ministries or its official Institution(s). The competition has to be conducted on the territory of this nation. The duration of the Olympiad (including the arrival and departure days) should not be less than 9 days.
English is the working language of the IJSO and competition problems and their solutions should be presented to the IJSO International Board (IB) and team leaders in English. The organisers may also prepare these documents in other languages.
Within five years of its entry in the competition a country should declare its intention to be the host of a future IJSO so that the order of countries willing to arrange the IJSO can be compiled. A country that refuses to commit to organise the competition may be barred from further participation.
In the event of the IJSO being cancelled, all participating countries including those countries that had agreed to send an observer will be invited to send full delegations to the following IJSO.
For more detail download IJSO Statutes.
Source: Official Website