There was a time when Mr. Rewrite ate Del Taco or Taco Bell almost every day, back when he was manning AP's 4 a.m.-noon shift in Honolulu and needed to grab something quick and cheap before collapsing after work. But eating cheap tacos shouldn't be an everyday thing, so Mr. Rewrite decided to expand his diet to include other types of fast food. Then his doctor said Mr. Rewrite's cholesterol level was about 5 trillion and his life expectancy was around 27, so he finally decided to acknowledge the existence of fruits and vegetables. Sort of.
Anyway, the ad above represents another newsprint-crinkling moment for Mr. Rewrite. Every day is an adverb (e.g. How often are tacos 39 cents at Del Taco? Every day.). Everyday is an adjective (e.g. Seeing botched references to every day/everyday shouldn't be an everyday occurrence. But it is.).
Del Taco might as well have said allday, everyday to be consistently incorrect.
Please excuse Mr. Rewrite. He's about to relive his Honolulu days by dropping $3.90 at Del Taco. Don't tell his doctor.