What's Inside The Letter That Transformed Zenitsu? [Spoiler Alert]

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,.

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.