10 In-Universe Star Trek Holidays (And When To Celebrate Them)
4. Starfleet Remembrance Day
Much like Remembrance Day and Veterans Day in modern times, Starfleet also has a holiday dedicated to honouring those who lost their lives serving the organisation. The main difference was that Starfleet Remembrance Day honoured civilians, scientists, and explorers who passed, as well as soldiers and veterans.
On this day, it's customary for Starfleet officers to wear a pin commemorating a ship they served on that lost crew or was destroyed. When the Enterprise crew observed the holiday in 2259 (in the episode Memento Mori), Captain Pike wore a pin for the USS Discovery, which was his only way to openly celebrate his friends who were sent to the future, a fact that was classified by Command.
We don't know for sure from the episode, but Starfleet Remembrance Day likely takes place on the same date as modern Remembrance Day and Veterans Day, that being the 11th of November in several countries.