To prepare for version 1.5.0 of Fire Emblem Heroes, the game will go offline between 26th June 2016 at 11 pm and 27th June 2016 at 1 am (UTC; times are approximate). No details about the contents of this update or its release date have been provided.
Additionally, while it was known to data-miners, the official Japanese Twitter has revealed that the Michalis Grand Hero Battle will be returning tomorrow. Presumably like other repeats, there will be related quests to earn more copies of Michalis.