The 'Mean Girls' star - currently in residence at Malibu's Promises Rehab Centre - had been planning to celebrate the milestone with a lavish party at Las Vegas' PURE nightclub on July 2.
The bash was called off yesterday (18.06.07).
PURE representative Kate Turner said: "We support Lindsay and wish her the best as she is taking care of personal matters at this time.
"Due to the unique circumstances Lindsay's birthday party at PURE Nightclub has been cancelled. We think the world of her and look forward to working with her in the future."