Answer:
Rabbits shouldn't eat celery every day. Celery, like most vegetables, is a treat food. Celery isn't very nutritious, and if a rabbit eats too much of it, the rabbit will "spoil its appetite" for healthier food (like hay). Dark-coloured celery greens can be offered more often -- they can be part of the rotation of leafy greens that go into the rabbit's daily "salad" -- but the celery itself should be limited to an occasional treat.
A healthy diet for a rabbit includes unlimited amounts water, of grass hay like timothy or orchard grass (for adults; alfalfa is better for rabbits under 8 months); as well as pellets, and a salad of fresh, dark-coloured leafy greens. Treats (fruits and vegetables) should be limited to a tablespoon or two for a normal, healthy 6 pound rabbit.
See the related question below for more information and links about a healthy rabbit diet.