The singer spies a couple "have a jovial treat" at hay-making time. After 20 weeks, "this fair maid fell a-sighing"; after 40 weeks, she is crying. She writes to her love. He rejects her, saying, "I dearly like my freedom."
Roud lumps this with Burns's "Corn Rigs" ("It was upon a Lammas night"). A source it may be, but Burns has done enough rewriting that I think they must be split. - RBW