There is a tip page in-game that explains how, though I can post it here too. In short: Amelia needs to have collected 4 essence and 25 in-game days need to pass
Might be pretty difficult. Good thing you don't need to to unlock Unwilling Fairy. Could take you 100 days to collect 4 essence, just need that moment where both are true and Amelia is outside to run into the Unwilling Fairy.
