M.P.
I would go. It's not like your son won't be able to go later.
I sympathize with you missing your son. However, I also think that you need to find a way to focus on other things too. Go and have a good time.
How is your daughter doing with you so focused on missing your son and feeling sad? Hopefully your mom is able to be more available for her.
I did not mean this in a cold way. I do sympathize. I have an autistic grandson and know how difficult life can be. I also know that when a parent focuses more on the autistic one then the rest of the family, life is more difficult for everyone. We have to find a balance. And one's birthday should be celebrated even when one member is missing.
I'm just thinking that because you said it's hard to bring myself around to do much of anything that you're focused so much on your son and your sadness that you may not be able to meet your daughter's needs. Also, because you don't want to celebrate your birthday with the family you have it sounds like you're not thinking of their pleasure when making your decision. Your post just sounds like you're mostly focused on your son.
You're right. Your daughter doesn't understand what is happening but she does feel your tension, anxiety, depression. You don't want to eat out with your husband, who has asked to take you, because your son can't be involved. What does your daughter, who cannot ask, want/need that you're not hearing/seeing? Just a question to be considered. If she's not acting out then I think her needs are being met.
Just a basic question based only on what you've posted here. If you're managing to include your family that is at home but not for your birthday, then I apologize. In that case, let me reword my question. Why not, go to dinner at this really nice steak house so that you and your husband can celebrate your birthday. Your son can go at another time. Enjoy this time which both you and your husband deserve.