Btvs This article is about a subject whose real name is unknown, and is known only by a title, nickname or alias.
The world is spinning. Straight, to the new day. Big day, big big day...
―Tara Maclay[src]

The "Big Day" was the term the Sunnydale population of mentally-ill humans employed to refer to the mystical alignment in which the hellish goddess Glorificus had to employ the Key in order to shatter the barriers between dimensions in order to return to her own. The Key would not work if the ritual was enacted at any other place or time. The place was a spot in the air reached by the Tower.