I agree with you and with everyone else on this board - a celebration of your mother's life is the right thing to do. You'll also be mourning and missing her, but what else would you do on her birthday? Would it be better to sit around alone thinking of her and being sad, or getting together with others to remember how special she was? My mother's 65th birthday was a month after she died, and a few special family members and friends got together at her house like we used to (it wasn't cleared out yet), we made a big dinner, had a birthday cake with her name on it, sang happy birthday (okay, that part really choked me up), and let my daughter release a balloon in the sky as a message of love to Mom. Then we watched video clips of her and laughed and cried. It was sad but it was joyful because it was the closest thing to having her there with us.
I felt that if we didn't acknowledge her birthday, it was like saying we didn't remember her and didn't care. It's a wonderful way to spend the day thinking of her in a positive way without being told you should get over it - just a day to wallow in your grief, your wonderful memories and share them with others. It's much better and more therapeutic than a funeral, which can be completely sad.
In the end, I always ask "What would Mom have wanted me to do?" and I know she would have wanted a big party for her loved ones, as Scragots said (happy birthday to you, Scragots!) So, what would your Mum have wanted? That's all that matters - and what YOU want, too.
I miss my mum so much, too.