From what I recall, the summer solstice celebrations were a mix of age-appropriate scientific explanations about the solstice and some studies about the sun and the Earth and plant/animal growth and life cycles, as well as songs and games and stories about the sun.
The science involved things like demonstrations with a flashlight or a lamp and a model of the sun and the Earth. Students also often had started seedlings or had been working in the school garden at this time of year. For pre-schoolers, the science was pretty basic and understandable.
Solstice often bumped up against the end-of-school year celebrations, so there was often a picnic or party with lots of seasonal food.
Solstice crafts included things like making masks in the shape of the sun or sunflowers.
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