Buffy Summers' birthday has two common themes throughout the series: whenever it is shown on-screen, it takes place in the twelfth or thirteenth episodes of the current season, and is typically followed by supernatural atrocities. It is not celebrated on-screen in the first or seventh seasons, ostensibly because the first season was only a half season, taking place after her birthday and because Spike advised her not to celebrate her birthday any further in the sixth season.
- The loss of Angel's soul and the return of Angelus ("Surprise/"Innocence").
- Buffy losing her powers in preparation for the Cruciamentum ("Helpless").
- Rupert Giles being transmogrified into a Fyarl demon by Ethan Rayne ("A New Man").
- Dawn Summers' discovery that she is the Key and subsequent identity crisis ("Blood Ties").
- All of the participants of Buffy's birthday party, including anyone else who entered the house, being trapped inside the Summers residence by Dawn Summers' unwitting wish to Halfrek ("Older and Far Away").