Since you both engaged, I would assume you have been together for awhile... If he does forget, be cool about it... He loves you and keep in mind... this is the craziest time of the year and people have a lot going on.. However, if he does indeed forget and evening comes (by this time you should be pretty irate), remind him, with a smile... remember, he loves you.. (by the way.. my husband of 10 years has a birthday on Jan 3rd and I think I only remembered half the time... he is really great about it too.. he understands the chaos and knows if he doesnt remind me, in time I will realize I missed it and he only gets more attention due to guilt..
You should not be mad at him because he does not forget on purpose. The best thing for you to do would be to keep dropping him hints and reminding him about your birthday before the 31st. It would be less likely for him to forget.