All Hallows' Eve
The Scotch tradition is, that those born on All Hallows' Eve have the gift of double sight, and commanding powers over spirits. Mary Avenel, on this supposition, is made to see the White Lady, invisible to less gifted visions.
“Being born on All-hallows' Eve, she (Mary Avenel) was supposed to be invested with power over the invisible world.” (See Sir Walter Scott: The Monastery, chap. xiv.)