"Catch of the day" is a phrase originally associated with seafood dining. The catch is usually a type of fish, often in abundant supply, being served to diners that day. One would hope this fish was newly caught and fresh; it is also frequent, however, for restaurants to advertise the fish they need to get rid of most quickly. Usually the catch of the day is posted on a sign somewhere in the restaurant; sometimes servers point them out or will list them at your request. Think "soup du jour," only fish.
"Welcome to Crabs 'R Us. My name is Mindy. Our catch of the day is flounder!"
Colloquially, the phrase has been used to imply any newly-acquired person or thing who is not likely to stay around for long.
"Oh, Mindy's that blonde girl from accounting. She's Jack's catch of the day."