Oh gosh, I can only imagine how you're feeling right now. That sinking feeling that the person closest to you has forgotten something so important is just awful. With two little ones myself, I know how much of our own stuff can get pushed to the back burner while we're making sure everyone else is taken care of, but a birthday is different.
I get the expectation that our partners should just know. But sometimes, life gets in the way. He's probably so caught up in organizing his mum's big birthday bash that his brain has just become a sieve for everything else. It doesn't make it right, but it's a possible explanation.
I know the temptation to just let it ride and see what happens is strong, but honestly, is it worth the potential disappointment? A part of me, the part that's a bit of a romantic, wants to say, "Let him surprise you!" But the realistic, tired mom-of-two part of me says, "Girl, just drop a hint."
Maybe a super subtle one? "So excited for your mom's party tomorrow, it's such a big day for her. It's crazy how close our birthdays are, isn't it?" or "I'm looking forward to a bit of a break this week... I think my birthday deserves a bit of a celebration, even if it's just a quiet one."
You deserve to feel celebrated. If the hints don't work, then maybe a direct but gentle reminder is the way to go. You don't want to spend your birthday feeling hurt or resentful. I hope he pulls through and you have a wonderful day, no matter what!