What is an opposite poem?

John Milton, for his M.A. Vivovaci, had to write poems praising two exactly opposite things. He chose to praise the Night and the Day. His poem praising the day was L'Allegro which means 'the cheerful man' and his poem praising the night was Il Penseroso which means 'the thoughtful man.' These two poems, though written while a student for his M.A. examination, are masterpieces in English poetry. It can be deducted from their lines that the great theme for the Paradise Lost was developed from them in those very early years.