Why am I not allowed to smoke a day before lasik surgery?

Smoking. Smoking is not contraindicated prior to lasik but it is terrible for you and surgeon probably does not want to smell it on you.
And after LASIK too. Smoke can increase irritation in your eyes. I usually ask my patients not to smoke for a few days after surgery. This is also not a 'rule', but a strong recommendation.
Dry eyes. Smoking can also decrease tear production, which can increase dry eyes. This is not a strict rule, but it will certainly help speed recovery if your eyes are more lubricated.
Many reasons. Smoking has been shown to increase the risk of infection related to surgery or contact lens use. It is also associated with a higher prevalnce of cataract formation.