昔年夕颜
[xī nián xī yán]
Evening Face of Bygone Years references a fleeting yet beautiful moment akin to evening flowers that bloom only briefly before dusk. Here '昔年'(xinián) meaning years gone by evokes nostalgia and transience, while '夕颜'(xīyán) refers specifically to moonflower whose bloom lasts but a short time into night.