Gotcha Day Alternatives

I've tried doing some research and haven't had much luck. I have two children adopted internationally. At one point I thought that would be my only two children. I now have a 2 year old step daughter too who is not adopted. Previously I had celebrated 'Family Day' with my two adopted children. I don't want the 2 year old to feel slighted when she gets older when she sees that for the other two children we celebrate their birthday AND another special day.Any ideas and alternatives? What do other mixed families do out there? Thanks