1. Public holidays are immaterial in correspondence from a lawyer because correspondence from a lawyer is not binding, meaning you don't have to reply within the time frame they give you, and in fact, you don't have to reply at all if you don't want, and there's not a thing they can do about that.
2. What your ex's lawyer chooses to charge your ex, is between your ex and your ex's lawyer. Don't worry your little head about whether or not they're allowed to charge for letters sent on public holidays. That's your ex's dispute to take up with them if she wants, not yours.