An opposition day is a day in a legislature using the Westminster System in which an opposition party sets the agenda. Most days the parliamentary agenda is set by the government; opposition days allow the smaller parties to choose the subject for debate. The number of days varies between parliaments. In the United Kingdom there are twenty opposition days per parliamentary session, and in Canada there are twenty-two. The days are divided among opposition parties with the Official Opposition allotted the most; In the UK, the Official Opposition party is given 17 opposition days, and the second biggest opposition party given 3 days. Canadian opposition parties are allocated days, which are also called "allotted" or "supply" days, in proportion to their membership in the House. There are no opposition days in Australian jurisdictions. In India, the Parliament does not have an opposition day, but every week, one day (every Friday) during a session, is devoted to Private members bills and resolutions.
Originally, opposition days were associated with debates over supply and held prior to the estimates and the budget and were created so that opposition parties could advance ideas for what should and should not be funded. In modern politics, opposition days are most often used to bring up issues that the government would rather ignore.
On 29 April 2009, the British Government was defeated in an opposition day vote on the subject of settlement rights for Veterans of the Gurkhas. This is the first time since January 1978 that the government has lost an opposition day vote.