Boise Ed submitted this Lockhorns comic, asking: “What is ‘not-leg day‘ supposed to mean?“
P.S. This submission accidentally fell through the cracks. At the time, Boise Ed reported that “Comics Kingdom has apparently been on holiday since at least last night, so I’ve given the ArcaMax URL“. As I’ve explained elsewhere, Arcamax-URLs have a very limited half-life. In addition, since this comic was published, Comics Kingdom has been completely rebuilt, and The Lockhorns have changed syndicates, so that the strip is now at GoComics. Luckily, Ed’s brief, but accurate description made it easy to verify the correct comic.
Comics I Don't Understand 
People who go to the gym on a regular basis often devote a day to a specific part of their body so on Mondays they concentrate on their arms, Tuesday their core, Wednesday their legs so Wednesday would be a leg day. While she is off to the gym he’s going to spend the day in front of the telly with his feet up. So not a leg day.
Ian has it. What’s strange is how oddly his leg is drawn. It almost looks like “not-leg” might refer to “an alternate non-leg limb, connected to the trunk in the same location, used in snaring prey.” But there’s no joke there.
What the other folks said, but also note that “never skip leg day” was a popular Internet meme from roughly 2004-2014 (judging by the Know Your Meme writeup). By Lockhorns standards, this is incredibly cutting-edge humor.
I just noticed that she appears to be carrying a yoga mat, for whatever that has to do with the case.
And a water bottle. Apparently Loretta was actually exercising.