I’m really surprised that this is the top post here.
It’s not just a noun and a day of the week. It’s a noun and a day of the week used to formally sell tacos.
This is Taco Bell using its size and wealth to take something from smaller players so that it could make money with it.
And the smallest players have always had absolutely nothing to worry about. Part of trademark law is that you must defend it. That can be just a letter.