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.
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. Typically, governments seek to amend opposition day motions to support their own policy.