Mine was when my big cousin got married in the 70s. I got one of those long dresses with the tunics underneath a la The New Seekers. It was a bottle green printed fabric with a bottle green high collared tunic. My sister had a brown version with a yellow tunic. Man, we thought we were gorgeous!

Our neighbor was a seamstress and she made them for us. We also got our hair pinned up with flowers in it. I guess I would have been about eight or nine.
It was like the red one in this pic:
http://www.fashion-era.com/images/1970s/tvpinafore.jpg