Rival chain Taco John's has held a trademark since 1989
That sounds pretty strong. Only in the internet era of making every single day a "thing", did I hear of Taco Tuesday. They certainly have a claim, but is it a claim on something they can rightly hold?
OTOH, never even heard of Taco John's before... so... small fish eaten by big fish soon?
Can the general public, via the internet, consume another's slogan and claim it as public domain?
Then again... if all Taco Bell wants is the right to say Taco Tuesday.... seems innocent enough?
If I were Judge over this case, I am not sure how it should go down.