An old folk belief in rhyme: wash your face in May Day dew from a hawthorn tree and you'll be handsome ever after. A nice way to introduce a real seasonal custom alongside the verse.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
The fair maid who, the first of May,
Goes to the fields at break of day,
And washes in dew from the hawthorn-tree,
Will ever after handsome be.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition