Melancholiday

Melancholy.
Holiday.
Melancholiday.

Days that are celebrated because they commemorate something sad, as opposed to something happy.

Taboo.
Boohoo.
Taboohoo.

Like Pearl Harbor Day, or Arbor Day if you hate trees, or even holidays that DON'T have the phrase "Arbor Day" contained somewhere within.

Botanical.
Calendar.
Botanicalendar.