The even was the time when all the lamps were to be lighted, 1 Samuel 3:3. See Poole on "Exodus 27:20", See Poole on "Exodus 27:21".
Related Commentaries of Exodus 30:8
Exodus 30:8
8 And when Aaron lightethd the lamps at even, he shall burn incense upon it, a perpetual incense before the LORD throughout your generations.