Surprise

In which Buffy has a birthday, Dru has a party, and both Buffy and Angel lose something.

From the old WB Buffy website:

PART ONE OF A NOT TO BE MISSED TWO-PART EPISODE -- The fate of the world is at stake when Spike and Drusilla gather the dismembered body parts of The Judge -- a demon with a deadly touch -- from the ends of the earth as their ultimate weapon to extinguish The Slayer. Meanwhile, Angel's ability to feel human emotion is in jeopardy when he and Buffy share an intimate moment which threatens to destroy his soul.

Cross-reference:

episodes featuring Buffy's birthday:

• Helpless
  Buffy faces her ultimate trial, and her ultimate betrayal, on her 18th birthday.
• A New Man
  Giles is transformed by Ethan as Buffy celebrates her 19th.
• Blood Ties
  Dawn goes missing on Buffy's 20th.
• Older And Far Away
  Buffy is 21, and throws the weirdest, possibly the longest, birthday party ever.
• this episode
  Buffy is 17, and takes her relationship with Angel to a new level.

episodes in which characters lose their virginity:

• Consequences
  Xander
• Graduation Day (part 1)
  Willow
• this episode
  Buffy

Cast:

• Sarah Michelle Gellar as Buffy
• Alyson Hannigan as Willow
• Nicholas Brendon as Xander
  

  ---

• Anthony Stewart Head as Giles
• Charisma Carpenter as Cordelia
• David Boreanaz as Angel
• Kristine Sutherland as Joyce
• Brian Thompson as The Judge
• Robia Lamorte as Jenny Calendar
• James Marsters as Spike
• Juliet Landau as Drusilla
• Seth Green as Oz
• Eric Saiet as Dalton
• Vincent Schiavelli as Uncle Enyos