What about "Flight day" , memmorating the escape from Earth ?
Or "Founders day" for first landing, or some sort of festival for each House ?
If any faction declares it´s own festival days thats up to the leaders I believe, but if there is perhaps a major festival day for each House I would greet it for RP reasons.
Liberty : "Founders day" for first landing.
Brettonia : "Queens Birthday" ?
Rheinland : Constitution day ?
Kusari : Ancients memory
...