Grendel, one of the monsters fought in the epic Beowulf, only attacks Heorot at night. This is significant given his aligning with evil and darkness.
Grendel is a descendant of Cain. Exiled from God's light for this ancestry, Grendel hates God and those who celebrate them. Since he is not able to walk in the light, a place of God, Grendel must walk in darkness. This concept shows his alignment with the devil (also exiled by God) and his natural feud with mankind and Beowulf.
Therefore, Grendel's attacks at night symbolize his position in darkness and his alignment with evil.
One could suggest that Grendel's appearance at night suggests the pushing of Pagan ideology into the past. By placing light on Christianity, and darkness on Paganism, Grendel's characterization illustrates the movement from Paganism to Christianity.