Depends very much on the person, I know someone who doesn't like to be reminded and someone else who wishes more people would mention their name & bring it up in conversation. Ask her what she would like.

Are you a bit special?

I am a widow myself. Absolutely do not wish her a happy anniversary. I have a friend who shares a birthday with my late husband. She used to say on the date, hope you're okay, I know today is hard. Some years I didn't even take notice of the date, it would stop me in my tracks and bring tears to my eyes. I know she was only trying to be nice, but it was upsetting. It's better not to say anything at all. I agree that it would be nice to invite her out on the day if you can, and be there for her if she wants to talk about it, but do not mention it.

How do you even remember the date of your friends anniversary?

It depends on the friend and the way they are which you do. Tread carefully. If they are the sort of person who would appreciate a bunch of flowers and a card saying “thinking of you” do it, but if they will feel upset by this don’t. Just do what you think is right.

Don't wish them "happy anniversary", but call/ text to acknowledge what a hard day it must be for them.

Invite them to lunch or dinner. If they want to talk they will and just give them comfort of being with someone

It's still their anniversary, I would call for a chat and hopefully if my friend wanted to talk about but they would.

Well no, because there's no longer an anniversary. Instead just say hey hope you're going OK today

I would ask if she or he would like to do something with you that day. I wouldn't necessarily acknowledge the anniversary.

I wouldn't acknowledge it like that, it wouldn't be a happy day to your friend anymore. It will be a difficult day so instead you could just let her know you're thinking of her and invite her out for a coffee or a few wines if she wants to talk.