This tradition stems from the old belief that witches were a their most powerful during Easter week and that they would travel to Blåkulla (Blue Mountain) via broom (or even ride stolen livestock like cows, goats, sheep, or even cats - see image below) to cavort with the devil on Maundy Thursday through Saturday.
To honor this old belief, Scandinavian children dress up as witches and go trick-or-treating, like Halloween! In some parts of Scandinavia children leave Easter cards or a small token of thanks to the houses they visit. Many of these cards feature images of påskkärringar, or Easter witches.
I love how you can find traditions similar to Halloween throughout the year for many different holidays. I love the Scandinavian tradition of Easter witches and it is fun to learn about different holiday traditions from around the world that relate to Halloween.
Glad påsk (Happy Easter)!