It’s May Eve, (April 30 also called Valpurgisnacht) the night when European children (especially in Germanic and Celtic countries) open their windows a crack to allow the fairies to come in and rest.
Why do the fairies need to rest? Because they are flying around (some on brooms?!) battling the forces of darkness, chasing away the last vestiges of “dark” powers of non-growth aka winter and the cold half of the year. On May Day the warm half of the year begins. At last.
Six months later to the day we always have another such battle when the “mean” fairies aka witches kinda win the sky battle. That would be Halloween of course.
If a fairy takes rest on your windowsill (because you left your window open for her) she will leave you a coin. It’s like the tooth fairy, you see, both are old Germanic traditions.