Its hard when things change in life...birthdays especially.
I feel for you...
having said that, to answer your question, "what would I do"; I'd do all my favorite things. ALL of them! I'd work to move past focusing on what I won't get, to what I can rediscover about myself, who I was as a girl, before I became a wife and mom...
I'd eat a tripple scoop icecream cone, I'd take a day trip someplace I love to go to, I'd hike, I'd listen to my favorite music, go to a movie and read my favorite book of comic strips! I'd paint and draw and color the patio in sidewalk chalk! I'd paint my toe nails or get them done. I'd buy myself a new dress and wear it all the day with a tiara! Haha! I'd wear a pink feather boa and dance with the dog.
In short, I'd be sad, but I'd have fun anway...and I'd ask for a "make up day" that could be spent celebrating my birthday with those I love most on a later day that works for everyone.