For anypony who has a special somepony on Hearts and Hooves Day; this holiday is the happiest time for them. For singles like Princess Luna; it's the complete opposite.
Being immortal severely affects the Royal Sister's choices of courtship since they will outlive any mortal pony they choose to marry. Celestia has accepted this fact and has chosen not to marry while Luna has not, and it makes her miserable whenever the holiday comes around, or is even mentioned.
One night though there is a glimmer of hope when she receives a letter from a secret admirer that doesn't care about mortality, or her checkered past. Instead they accept her for who and what she is.
Hopefully this will be what Luna needs in order to get into the holiday spirit.
Cover artwork by JohnJoseco and W0lflicious