WARNING: The following contains spoilers for Legends of the Dark Knight #9 by Brandon Thomas, Giannis Milonogiannis, Jordie Bellaire and Deron Bennett, on sale now.
For the most part, the villains of Batman's rogues' gallery are some of the most grounded in the DC Universe. Though some of them have meta-human powers like Poison Ivy or Clayface, most of them are simply psychotic individuals or ambitious gangsters. The magical and cosmic threats faced by the Justice League don't tend to intersect with Batman's war on crime. However, one classic villain, the Calendar Man, just changed that.
In Legends of the Dark Knight #9, Batman saves three children from a burning blimp, only for the blimp to explode in a very strange way. The explosion spells out a letter of the Martian alphabet. Batman recognizes that it's the eleventh. The kids find that strange, as it's all their birthdays today and all of them just turned 11. With this attention to dates, it doesn't take Batman long to realize that he's dealing wiht Calendar Man.
Calendar Man first appeared in Detective Comics #259 by Bill Finger and Sheldon Moldoff. He was one of many comical Silver Age villains and had a multitude of seasonal costumes to reflect his ridiculous date-orientated crimes. Over the years, Calendar Man has conducted many different types of calendar-based crimes, from crimes based on the seasons, specific holidays and even just the seven days of the week. However, one thing all of these crimes had in common was that they were based on calendars from Earth.
Batman calls in fellow Justice Leaguer J'onn J'onnz, the Martian Manhunter, for help with the Calendar Man case. Batman recognized the Martian letter from the Watchtower's databases but is far from fluent in the Martian language. Therefore, calling in an expert in Martian culture is the obvious next step.
Showing J'onn the letter Calendar Man left, J'onn recognizes the script but tells the Dark Knight he hasn't seen it in decades, not since they switched alphabets during the Martian Silver Age. J'onn then reveals something else about this particular version of the Martian alphabet, something which may cause problems for the Dark Knight. The original architect of the alphabet Calendar Man is using hid magic spells within it. If Calendar Man is now using a Martian calendar with this particular alphabet for his crimes, Batman may have bigger problems than simply figuring out when the criminal will strike next.
Calendar Man has evolved a lot over the years. With his first appearances in the Silver Age, he wasn't considered to be much of a threat, with his themed crimes and ridiculous outfits. However in the Modern Age, particularly in Jeph Loeb and Tim Sale's Batman: The Long Halloween, Calendar Man has evolved into a much more serious and far more dangerous villain. Batman: Rebirth #1 by Scott Snyder, Tom King, Mikel Janín, Mikel Janín, June Chung and Deron Bennett even saw Calendar Man turned into a metahuman character who changed with the seasons, dying in winter and being reborn in spring.
This could be another evolution of Calendar Man then. It's unlikely that this story will mention his metahuman powers from Batman: Rebirth. The more likely scenario is that in dabbling with this Martian calendar, Calendar Man will come across some of the spells J'onn mentions. This could lead to a Calendar Man imbued with cosmic magic, something that Gotham doesn't see too often.
It's a good thing that Batman called on the assistance of Martian Manhunter. The idea of a sadistic Calendar Man with magic on his side seems like something the Justice League may be called in to handle. Having a member of the League on hand to assist Batman in such a matter, as well as J'onn's familiarity with Calendar Man's newest cosmic obsession, makes him exactly what Batman needs right now.