She's 17 now.  You can let DH off the hook for parenting on this issue.  

celebrating those who gave kids life and who are their primary care givers through childhood.

Certainly, while it is children that qualify someone as a mother, the day has nothing else to do with children.

I honor my mother on that day, I honor my wife on that day, and I honor friends who are mothers.

I do not really give children a thought on that day other than start reminding my kid about a week ahead of time to not forget to call his mom.

Then don't make her your focus.  

You can't control what she does.  You know making a big deal to your DH about "not" reminding her will only cause a conflict.. so just take the path of least resistance.

When you get "the text" treat it like a fart... simply delete it..or open and close it without reading (to get rid of the notification).. then move the heck on with your day.  Just ignore it.. like you would a fart on the train.. what else can you do.. you can't stop the other person from farting right?

If your DH asks if you got it.. "yeah.. I think so.. oh.. honey.. we need to go back to that lunch spot I went to.. the chocolate cake was soooo good"  

I understand she is a bitter pill to swallow for you.. but a text literally has no power over you unless you allow it.

Perhaps you can ignore the text, and the person it's from by remembering that no matter how old she is your DH is teaching her manners, and very obviously showing SD that HE honors your position.  Him reminding her to wish you a Happy Mother's Day is her being reminded every year that he feels you are worthy of the attention, the honour and the gratitude of a caregiver.  The fact that she can't do it well reflects on her.  The fact that he insists every year that she do it reflects on him.  I'm sure she hates every year that your DH insists that you be acknowledged as a wonderful woman.  I'd acknowledge the text with a quick "thank you", and move on with my day.

Also, realize that 17 year old girls can generally be very self-focused.  It may not be an outpouring of emotion, but perhaps she really does mean HMD?   You know your relationship best.  

You can tell your DH for mother's day he can get your the gift of "butt the f@ck out."  Tell him you would like him to leave your relationship with SD between you and her.  I finally had to do this with my DH and I had to explain to him nothing from his Spawn was better than a fake anything.  

St. Thomas NV (Ghost Town) and Red Rock Loop Trail Lake Mead National Rec Area.

As I am writing this, I am not one of the recently discovered steel container encased human remains that were discovered this weekend as Lake Mead recedes due to the ongoing drought.

I must be doing something right to keep my bride happy enough to not dispose of my body.   So far.

A doozy...I would much prefer a text message and no phone calls from one of the adult SSs. My birthday and Mother's Day is the build up for "this call" and I would much prefer to focus on me that day. Typically the one adult SS says "I'll call her at this exact time" which in turn DH tells me about. I then have that occupying the back of my mind and then the call doesn't actually happen until the end of the day. I have shut down any facetime activity- I am working towards just getting it to be a text message from him. I've already set the example - I only send an obligatory text message when I must. This only goes for the one adult SS and the DIL. Other SS I kind of enjoy his messages- he's kind. I am 100% ok with having different interactions pending on the child- I do not play the game of even and balanced after all the crazy making from the one kid.