So sweet the hour, so calm the time,
I feel it more than half a crime,
When Nature sleeps and stars are mute,
To mar the silence ev'n with lute.
At rest on ocean's brilliant dyes
An image of Elysium lies:
Seven Pleiades entranced in Heaven,
Form in the deep another seven:
Endymion nodding from above
Elysium, or the Elysian Fields, is an afterlife equivalent to heaven in Greek Mythology. Those who are heroes or specially chosen by the Gods are allowed entrance there after death. In the Elysian Fields, one is allowed to indulge in any pleasures they desire for eternity. 

Serenade