I agree with Breton. If you do have contact , just say "happy day", and leave it at that. This will probably make more of an impact on his ego than anything else. Somewhere in that head of his , he hasn't connected that being a father is not simply conceiving a child, but also sharing in that child's life while they're growing up. Yes, he is a father..but he is a AWOL father. Perhaps in time he'll start to act more like a father than just a sperm donor, but for now, don't worry about 'celebrating' the day for him. If the kids want to contact him, that's something different..but you are not obligated to let him know that you're thinking of him.
I know exactly what you meant when you said how hard it is NOT to be able to give what you feel towards someone you love. It's a special kind of pain, isn't it? All that love, emotion and sentiment..and nowhere and no one to share it with. Been there, done that. After all these years, it still stings quite badly, but I've survived, and you will too.
I guess the one thing that I can say is that probably like we think of them on special occasions, we also cross their minds on those same days. That you can be sure of.
I'm happy to hear that your kids are setting up fun things to do this summer. I hope you've got some enjoyable things planned for yourself too.
Women are angels. And when someone breaks our wings, we simply continue to fly...on a broomstick. We are flexible