That sucks and I’m sorry you’re so down about it.
I have two not entirely aligned thoughts on your situation:
1. Tell your friends directly that you understand that they are busy, but that this is very important to you and you are really sad that they can’t make a little time for you on this important day. Put it out there “I’m disappointed and sad that you can’t make time for me on this important day. I know that you aren’t responsible for my feelings, but it seems you’re putting in no effort on this and it really makes me sad to know I mean so little to you.”
2. Is there any – and I mean any – chance that they are planning something and all of this is some ruse to make you think they don’t care? Because while people may think “Hee, we’ll make her think we don’t care and then we’ll surprise her to show how much we care” is fun – it’s cruel.