You're lucky that you only have one!! Back in highschool, my birthday was always right in the middle of exam week. Oh did it suck.
Go out and party after your exam.. that way you won't feel guilty on your birthday for not writing it.
just study really well for u test before ur b-day and then u won't have to worry about it ... and that will just make u birthday even better if u know u did really well on ur exam
go take ur exam..after ur done..u'll have that big relief as well as the excitement of partying it up for ur bday...it'll be fine..u'll be looking so much more forward to the partying that the exam will fly by...have fun with the exam!