The elders are far more alert for an early lunch party. The assisted living facility should have a shuttle that can accommodate her fellow residents with advance planning. On site or at a local restaurant, include her closest/favorite caregivers in the head count. They will sincerely appreciate the thoughtfulness.
A champagne toast to your Mom with lunch and your mother's favorite birthday cake are very appropriate.
We shall use the assisted living private party room. Mom refuses to leave the building, so an off site location is out of the question.