Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder hashira, jigoro.
Webduring the hashira training arc, a bird brings zenitsu a letter explaining what happened, and shortly after we see him being all serious.
Though if i remember correctly it's not.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder.