Twilight is the time just before evening, between daylight and darkness, when a soft glowing light comes from the sky. The light is reflected from the sky when the sun is below the horizon. 
 
In Henry Bunner’s poem “The Heart of the Tree”, the twilight is called ‘happy’ because we can hear the humming of birds living on the tree at this time, making us happy.

For song and mother-croon of bird
The hushed and happy twilight heard —