A Letter to Amy (1970)

"Peter [from Keats' The Snowy Day] is having a birthday party, and he's asked all of his friends to come. But Amy is a special friend because she's a girl, so Peter decides to send her a special invitation. When he rushes out in a thunderstorm to mail it, he bumps smack into Amy herself and knocks her to the ground. Will she come to his party now?"