What would you do …….?

No - say nothing. Least said soonest mended. Try not to fret about it .

My experience of estrangement is that it makes you anxious about all relationships. I'm sure your son just forgot - there is nothing more to it than that. Don't say anything - that could just cause tensions that don't currently exist

I understand your sadness and hurt, BUT, say nothing, just endure your son and family are all well and stable and move on as if nothing has happened

No.
Easy to forget.
Especially if there are two dates to remember.

It is not important. It’s just one day. It is how he behaves the rest of the year that is important.

One of my sons forgot my birthday a while ago. Decided to let it go but felt a bit hurt. Everything fine since and he has sent cards and presents since. Just a lapse and not worth alienating your son by mentioning it.

I would be hurt too.
On one of my birthdays, my youngest son gave me a lift to work, because he wanted to borrow my car. He never said ‘happy birthday’ - I thought he had forgotten and shamefully reprimanded him and almost sure flicked my hand at him. I still feel bad about it He was just pretending. And, of all my three kids , he is the most placid and the sweetest.

I forgot my wedding anniversary this year of 45 years. I've just not been well suffering with vertigo and it completely slipped my mind. It happens.

My DSon has always either completely forgotten my birthday/Mother’s Day or turned up a day late with a card and present.
It makes me smile and shake my head, as I love him to bits and I know he loves me and that’s all that matters.
I’m sure the same applies to you Op, so my advice is to not say a word but smile and shake your head.

PS - now that they’re married, the wives remind them 😂

About 35 years ago husband and his family totally forgot my birthday. It was a genuine mistake. Our children were too young to realise. I said nothing and just celebrated it at work, displaying cards there. About a week later M-i-l realised and was mortified. Asked my husband was I upset he was doubly mortified. It all worked in my favour as my birthday was never forgotten again and I very graciously 😁 didn't mention it again.

Let it go....busy lives and easy to forget a day when there are two and you live in different countries.
You will also be quite sensitive to this as you are already estranged from one son.
It happens, so just keep your mind busy on other things. Open communication with him is something you can appreciate every day.

As eddiecat has posted Granless, when you're living with estrangement from your adult child you understandably worry about your relationship with your other child(ren), especially if you don't know what caused the estrangement.

A missed mothers day or even a birthday can seem far more significant than it needs to be .

Say nothing. I can't imagine why you care about a card.

Thank you one and all especially ‘Smileless’ who seems to understand where I’m coming from - ‘Norah’ doesn’t.