I think something to mark the end of slavery is a worthwhile national holiday. In my opinion, though, I would take MLK day and really lean into it as civil rights day. That way you could include Other civil rights movements, including women’s right to vote and honor them as well.
Personally, I would eliminate Presidents’ Day and replace it with a constitution day. No reason we should celebrate just one branch of government make constitution day the same day as election day and federal holiday.