15yo dd didn’t call on my birthday

She texted.
She's 15 and with mates at camp. Why would she miss you?!
No teenager wants to have to shoulder the burden of parental neediness.

I don't think I have ever, in 60 years, telephoned anyone on their birthday, nor they me. Is it a thing?
Yes it's most definitely 'a thing'. In my experience in pretty much every family I know.

I'm over 60 and it's a thing in my (immediate) family (and we are pretty laid back about things like birthdays). My siblings and I would always speak to our parents on their birthdays. I can remember standing in phone boxes decades ago calling home when I was a student.

I wouldn’t think twice about this. She sent a text which is great.

she is a teen enjoying a bit of independence and not always thinking about home. Let her enjoy that.

This, I would expect a teenager in a summer camp to just get on and enjoy themselves, I'm sure she'll make it up to you when she returns.

Whilst I agree the aim of parenting is to raise confident, independent teens, I'd add thoughtful and considerate to that list. Making a call on your Mum's birthday is not a big ask nor a massive imposition even when away at camp.
If your dd realises that, and genuinely apologises, showing she understands why you were hurt by the lack of a call, then you know she's heading in the right direction OP. If not, I'd explain that you were sad that she hadnt called.
They're still learning as teenagers so I wouldn't make a massive deal of it. But if parents don't guide them, how will they learn to consider other people's feelings as well as their own?

She sent a text - what else do you need? I don't think I have ever, in 60 years, telephoned anyone on their birthday, nor they me. Is it a thing?
Your children sound fantastic, and you have brought them up to be confident and sensible. A child constantly phoning would worry me hugely as being clingy, lazy or somehow incapable.

I think it's pretty normal for this age. They have to go through a separation phase from their parents in order to grow into independent adults and this can manifest in thoughtless behaviour like this. It doesn't feel nice but it's better than ending up with a 25 year old that isn't capable of making a life of their own.

Teens can occasionally be a bit thoughtless particularly when out having fun. It sounds like you have happy well-adjusted kids but perhaps haven’t got enough life of your own or time with DH. Focus on that in this next decade of your life.

My thoughts exactly. Mother’s Day and birthdays are much of a muchness, and I say that as someone who likes a celebration as much as the next normal person. But if my DC is off having the time of their lives and forgets to send me a card or a text, that’s fine.

I’d like to think I’ll be taking advantage of them not being around and having too much fun of my own to even notice.

Does she live with you?

She's at camp, I doubt there's much wiggle room for calling parents. I would give her a pass on this one

I think it's completely normal, and healthy for teens to not be in touch with their parents at sleepovers or when they are away. It means she's able to function alone, and isn't sitting on her phone all the time with her friends. Personally I think this means you've done something right, no teenager should be having to ring their parents everyday when they are away from home

You haven't made mistakes. You've created confident kids who are getting ready to make their way in the world. Having them constantly call you isn't a good thing.

It's a good thing they don't miss you, as hard as that feels. You've done the job you set out to do.

I feel so upset for you. This could so easily happen in my (very loving) household. We do so much for them and they can be so thoughtless. My adored dd who I am so close to didn’t get me a Mother’s Day card and I felt so sad despite trying hard to think I didn’t mind. I did. That of thoughtless of your daughter and you must pull her up on it, as I abandoned putting on a brave face and told her directly she had hurt me and I deserved better. Your daughter will love you so much but does not reminded to be thoughtful. Wishing you a happy 50th.