If you want to challenge the undisputed industry leader, make sure you get the timing right, and try not to draw the sort of attention that might make people take a closer look at your professed values. Shake Shack, a New York City-based fast food chain, announced on Thursday that throughout the month of April it will offer a special Sunday promotion featuring its chicken sandwich. The announcement included language clearly intended to needle Chick-fil-A, which does not open on Sundays. Also on Thursday, Nation’s Restaurant News reported new 2023 sales data that showed that last year, the Chic…