i know all this will sound harsh, but surely the people who died, whether friend, family or stranger, would feel bad if you put your celebration on hold because of their death?
I know if anything happened to me I would want my friends, family and aquainences to enjoy their own lives and not worry about how I might have felt. Sure it's sad and upsetting, but life must go on. Remember them with fondness and maybe a little sadness, but never hold your life back because of their misfortune.
Mel, your Dad surely wouldn't want you to mourn for him for the rest of your mum's life would he? You have to remember him, but not the sad things. Remember the happy things and use your birthday to celebrate not only your birth but your dad and everything nice you remember about him.
And the same for you FJWH, remember your friends life and the fun times. You'll never forget him/her now.
Hope this sort of makes sense at this sad time for you.