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HAPPENING 29 JUNE 2026 | More than 320 childcare locations, schools, after-school care sites and nature playgrounds in Zuid-Holland will take part in ModderDag (International Mud Day) on Monday 29 June. The annual mud day gives children space to play freely with mud, sand and water, from baking mud pies to building castles, canals and dykes.
Image: Children play in the mud during ModderDag. Photo: © Sharallya Egger.
The idea is simple, messy and probably a nightmare for white trainers. Children are encouraged to feel mud under their feet, use their hands, get wet, get dirty and discover nature through play.
Mud play gets children outside
ModderDag is organised each year on or around 29 June. In Zuid-Holland, more than 320 locations are joining in this year, mostly childcare centres, alongside schools, after-school care locations and nature playgrounds.
For families in Rotterdam and the wider region, the day taps into a familiar concern: children spend more time indoors than many parents would like. Research by Jantje Beton shows that 44% of children play indoors more often than outdoors during their free time, while three in ten children never or almost never play outside without an adult nearby.
IVN Natuureducatie and partner Humankind Kinderopvang en -ontwikkeling want ModderDag to push back against that trend. The day puts free outdoor play with water and sand at the centre, not as a cute extra, but as something children need for healthy development.
“Where playing outside in nature used to be self-evident, it is under pressure for more and more children,” says Ian Mostert, project leader for nature at schools and childcare at IVN Zuid-Holland.
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Nature play supports development
Playing with mud, water and sand is more than a chaotic photo opportunity. During outdoor play, children come into contact with fungi and bacteria found in soil and on plants, which can help support their immune system.
It also matters for sensory development. Mud has texture, weight, smell and temperature, and children learn by touching, shaping, slipping, digging and experimenting with it. Very inconvenient for laundry, yes. Quite useful for childhood, also yes.
The organisers emphasise that this kind of play helps children build both physical and mental resilience. By using their whole body, children learn how nature behaves, how materials change and how they can move through an outdoor environment.
“Playing outside simply makes children happier,” says Mostert. “What could be more fun than climbing a tree or playing in the mud?”
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Locations can still join
ModderDag is organised locally by participating organisations. Childcare centres, schools and play locations in Zuid-Holland that want children to spend more time in nature can still register. Organisers receive a free ready-made package with inspiration and practical tips to help them set up their own ModderDag. Registration is available via https://ivn.nl/modderdag.
The day is part of International Mud Day, which began in Nepal and Australia from the idea that children around the world should have more contact with nature. It is now marked every year on 29 June in dozens of countries. In the Netherlands, ModderDag was first organised in 2013. IVN Natuureducatie and Humankind use the day to encourage childcare locations to make more room for nature-rich play.
