The Mathematics Days are annual mathematics events for secondary school students. All events focus on skills such as problem solving, modeling, reasoning, arguing and working together.

Mathematics is more than performing procedures and also includes higher-order thinking skills like mathematical problem solving, creativity, reasoning, modelling and communicating mathematics. To assess these skills in an authentic way, the Mathematics A-Day, a competition for teams in upper secondary school, was designed in the Netherlands. Shortly hereafter a Mathematics B-Day was developed which showed that open-ended tasks for teams can also be designed within the domain of pure, formal mathematics.

Currently the Mathematics B-day and Mathematics A-Day take place in the Netherlands, Belgium, Germany, Japan, Denmark, Iran, and Slovakia. Organization of these events is taken care of independently by each of the countries. Participation in the international final of the Mathematics A-Day is open for all participating countries and organized by the Dutch committee.

The Math Day 15, for lower secundary education does not have an international contest and website, but the tasks are available in English.