The idiom " a red-letter day" meansAa day of bloodshedBa very important dayCa day of martyrsDan unhappy occasionAnswer: B. a very important day Read Explanation: In old calendars, important or festive days (like holidays or saint's days) were marked in red ink — hence the term "red-letter".Today, the phrase refers to any day worth remembering for its joy, success, or significance.Examples:The day I got my first job was a red-letter day in my life.Their wedding anniversary is always a red-letter day for the family. Read more in App