Possible Clues:

• This day is memorable due to scarlet missive
• Memorable
• Memorable, as a day
• This day is memorable - hence scarlet missive
• Memorable day to recall Hawthorne work
• Sounds as if one studied the missive for the day that's important
• Day of joyful occurrence when final reminder arrives?
• Scarlet missive for memorable day
• Sounds as if one studied one's correspondence for an important day
• This day is memorable for scarlet missive