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-lympiad, 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-lympiade 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 finale of the Mathematics A-lympiade is open for all participating countries and organized by the Dutch committee.

For more information you can go to the English websites:
https://www.uu.nl/en/education/mathematics-a-lympiad
https://www.uu.nl/en/education/mathematics-b-day

The Mathematics day for lower secundary education (OnderbouwWiskundeDag) does not have an international contest and website, but the tasks are available in English. For primary education we organize a similar event: The Big Mathematics Day.

Publications
Medová, J., Bulková, K. O., & Čeretková, S. (2020). Relations between Generalization, Reasoning and Combinatorial Thinking in Solving Mathematical Open-Ended Problems within Mathematical Contest. Mathematics, 8(12), 2257. https://doi.org/10.3390/math8122257

Bulková, K., Medová, J., & Ceretková, S. (2020). Identification of Crucial Steps and Skills in High-Achievers’ Solving Complex Mathematical Problem within Mathematical Contest. Journal on Efficiency and Responsibility in Education and Science, 13(2), 67-78.
https://doi.org/10.7160/eriesj.2020.130202

Wijers M., de Haan D. (2020). Mathematics in Teams—Developing Thinking Skills in Mathematics Education. In: Van den Heuvel-Panhuizen M. (eds) National Reflections on the Netherlands Didactics of Mathematics (pp. 15-29). ICME-13 Monographs. Springer, Cham.
https://doi-org.proxy.library.uu.nl/10.1007/978-3-030-33824-4_2

Doorman, L.M., Bos, R.D., de Haan, D.L., Jonker, V.H., Mol, A. & Wijers, M.M. (2019). Making and Implementing a Mathematics Day Challenge as a Makerspace for Teams of Students. International Journal of Science and Mathematics Education, 17 (1), 149-165.
https://doi-org.proxy.library.uu.nl/10.1007/s10763-019-09995-y

Doorman, L.M., Drijvers, P.H.M., Dekker, G.H., van den Heuvel-Panhuizen, M.H.A.M, de Lange, J. & Wijers, M.M. (2007). Problem solving as a challenge for mathematics education in The Netherlands. ZDM – Mathematics Education, 39 (5-6), 405-418.
https://doi-org.proxy.library.uu.nl/10.1007/s11858-007-0043-2