Saint Agnes' Eve

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun January 20th, considered especially in the British Isles to be the night on which young women dream of their future husbands.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The night of January 20, on which traditionally, if she performs certain rituals, a woman is supposed to have dreams of her future husband.