I took this shot through the car window, hence the odd angle. The sign doesn’t mean the Dodger Dogs that belong to Monday cost a dollar, in which case the apostrophe would be correct. It means that on Mondays (plural) Dodger Dogs cost a buck.
Some people have an uncontrollable urge to add an apostrophe to words ending in S, when most of those words are going to be simple plurals, not possessives. I’m just thankful “Dogs” didn’t get an apostrophe as well.