awe im sorry to here that but your birthday is tomarrow so Happy Birthday focus on the people who are there for you and say happy birthday to you and have fun with them and enjoy the company of people who do care and loves you dont focus on the negative and get a group of friends together go shopping out to dinner and party and get all dressed up and take pics and show your ex what hes missing out on stay strong enjoy your birthday:)
Find joy in yourself. You cannot depend on others for your happiness. That is your responsibility and yours alone. If you don't know how to make yourself happy, how can you expect someone else to do so. Put on your big girl panties your high heel shoes and your favorite red lipstick. But to answer your question more directly, as a woman, the is no better time than when I am with good girlfriend.
Do what you normally do on your birthday, but if I was in your shoes, I would stay in. If you go out, you'll seem desperate & trying to get back at him. Be the good girl & Don't do anything that you will regret. Boys come and go. You're gorgeous, you'll find yourself someone who appreciates and loves you, for you.