Why, in 1952, did department stores, notably Marshall Field, close early on Monday evenings?

To let employees go home early to watch the "I Love Lucy" show and for them to avoid the crowds going home to do the same.

"Marshall Fields even began closing early on Monday nights because they didn’t want to contend with the I Love Lucy crowd."

"Emmy-winning comedy I Love Lucy (1952-1957) starred husband and wife Desi Arnaz and Lucille Ball and enjoyed such popularity that some businesses closed early on Monday nights in order to allow employees to hurry home for the show."

https://en.wikipedia.org/wiki/1950s

Yes! And not just for employees--the public was staying home to watch "I Love Lucy" rather than going out to shop.

"I Love Lucy"...was so popular that, as Lucille Ball commented in her memoirs, “Our show changed the Monday-night habits of America. Between nine and nine-thirty, taxis disappeared from the streets of New York. Marshall Fields department store in Chicago hung up a sign: ‘We love Lucy too, so from now on we’ll be open Thursday nights instead of Monday.’ Telephone calls across the nation dropped sharply during that half hour, as well as the water flush rate, as whole families sat glued to their seats.” Marshall Fields specifically changed their weekly sale event from Monday to Thursday night.

http://www.theatlantic.com/magazine/archive/2003/09/-loosie/302787/
https://books.google.com.au/books?id=jT9gCwAAQBAJ&pg=PA47&lpg=PA47


lucy-desi.com/news/ill_facts.html and theatlantic.com/doc/200309/simpson no longer exist