From Fire Emblem Wiki, your source on Fire Emblem Wiki information. By Fans, for Fans.
Since 2017 Intelligent Systems has released monthly calendars associated with Fire Emblem Heroes. Production of new calendars are still ongoing as of writing.
The initial 2017 set of calendars was based on the top placing male and female characters in the 2017 running of Choose Your Legends. Not all of the 2017 calendar's art is taken from heroes; a handful of characters use their artwork from their source games. This does not appear to be the case with the 2018 set of calendars, which is instead based on the corresponding monthly Legendary Hero and other variant characters. In the 2017 set many of the calendar images came in pairs, one for the male character and one for the female character in the Choose Your Legends placement; this is a less common occurrence in the 2018 set.
- Four characters in the 2017 set of calendars weren't actually available as units in Heroes at launch. The characters were: Ike, Eirika, Ephraim, and Micaiah. The number increases to five if both Path of Radiance and Radiant Dawn Ike are counted separately.
- Neither variation of Ike nor Micaiah were part of the game at the time of their respective calendar image's release. Path of Radiance Ike, on a March 2017 calendar, wasn't part of the game until late April 2017; Radiant Dawn Ike, on a July 2017 calendar, wasn't part of the game until late January 2018; Micaiah, on a November 2017 calendar, wasn't part of the game until mid-January 2018.