Suppose it's late afternoon or early morning and the moon is visible -- can I say it then? Or must it be at "night"? (If so, how do we define "night"?)
The Rema 426:1 says it must be night when "the moon is shining and (people) benefit from its light". The Mishna Berurah exludes bein hashemashos and equates benefiting from its light with the time that the moonshine is detectable on the land.