I did a little research and discovered that there are actually two different definitions for the term blue moon. According to Wikipedia.org, the traditional meaning is the third full moon in a season with four full moons. However, the explanation of why there would be a special name for that event is a bit complicated and not one you would want to explain to your students.
Fortunately, the more widely-accepted meaning is that a blue moon is the 2nd full moon in a calendar month with two full moons. It's a fairly rare event that happens only once every few years. No one knows exactly how the phrase "once in a blue moon" came into existence, but it makes sense because blue moons are so infrequent.
Blue Moon Writing
Since a blue moon will occur tomorrow, I thought you might enjoy teaching your students about blue moons and having them explore the topic in writing. I created this Blue Moon Writing printable for you to use in conjunction with that lesson.
When you teach the lesson tomorrow, I would suggest starting with an announcement that today is a special day that happens "once in a blue moon." Then share some basic information about blue moons and discuss the meaning of the phrase, "once in a blue moon." Ask them if they've heard this phrase before and how it was used. Then brainstorm ideas for things that might happen "once in a blue moon" in their personal lives, such as staying up past midnight or traveling to a theme park. The writing topic is presented as a sort of "free write" expository assignment that can be used as a part of a science, social studies, or writing lesson. You can download this freebie from the Writing file cabinet page on Teaching Resources. I hope you enjoy it!