My headcanon for Sithmas (which may or may not coincide with Life Day, and prooobably has another official name) is that it's the only day of the year when Sith stay home. Traditionally-minded ones are supposed to not see anyone except close family and servants; the younger or more gauche ones tend to join the rest of Imperials in disguise (or "in disguise") so they can share in the partying.

It's 63 years since any Sith was assassinated on Sithmas. Doing so is social and quite probably literal suicide; some very powerful Sith are VERY protective of their one real day off.