Helpless

In which Buffy has a birthday. Her gifts include a psychopathic vampire, betrayal, drugs, and a deadly pop-quiz. Maybe people should just get her gift certificates.

Official back-of-box synopsis:

As Buffy's 18th birthday approaches, she begins losing her powers. When she finds out from Giles that it is a test that she must pass at this age, she feels betrayed by him. As she struggles to regain her powers, Giles is fired for not being an impartial Watcher.

From the old WB Buffy website:

COMING OF AGE - For her upcoming 18th birthday, Buffy eschews her friends' offer of a big party, hoping to celebrate with quiet reflection and a traditional trip to the ice show with her father. But unbeknownst to Buffy, preparations are being made for a life-threatening rite of passage that drains the slayer of her powers and then entraps her with a powerful vampire foe that she must defeat in a defenseless state in order to pass the test.

Cross-reference:

episodes featuring Buffy's birthday:

• Surprise
  Buffy is 17, and takes her relationship with Angel to a new level.
• 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 faces her ultimate trial, and her ultimate betrayal, on her 18th birthday.

episodes featuring the watchers' council:

• Who Are You?
  They imprison Faith, only it's Buffy, so it's lucky they suck at imprisonment
• Checkpoint
  Glory's a god, so what? Where's Gile's back-pay?
• Never Leave Me
  The council falls victim to a particularly cheap-looking explosion
• this episode
  They give Buffy a vampire serial killer for her birthday

Cast:

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

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• Anthony Stewart Head as Giles
• Charisma Carpenter as Cordelia
• David Boreanaz as Angel
• Kristine Sutherland as Joyce
• Seth Green as Oz
• Jeff Kober as Kralik
• Harris Yulin as Quentin Travers
• Dominic Keating as Blair
• David Haydon-Jones as Hobson
• Nick Cornish as Guy
• Don Dowe as Construction Worker