ETYMOLOGY
from seven + the plural of night
EXAMPLE
“… Among other, none was either more grateful to the beholders, or more noble in it selfe, then iusts, both with sword & launce, mainteined for a seuen-night together: wherein that Nation doth so excel, both for comelines and hablenes, that from neighbour-countries they ordinarilye come, some to striue, some to learne, some to behold. …”
From: The Countesse of Pembrokes Arcadia
By Sir Philip Sidney, 1590
Lexicophilia 