Movies
Put on your Sunday best, kids, we're going to Sears!
— Mike Brady ‧ The Brady Bunch Movie

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Mike gives a department-store trip the dress code of an important occasion. That mismatch supplies the joke: Sunday best sounds grand beside an ordinary shopping destination. His announcement makes a routine family errand feel like an event requiring ceremony and preparation.
Context reviewed