Well, this was a tradition that my father began when my sister and I were little. I was born 9 days before Christmas and my sister was born 2 days after. We tried having parties, but people were out of town, money was tight for others, the weather was crappy, etc. So, my dad started having an un-birthday for us on the first day of summer. We had a blast!!
Our original birthdays were never forgotten. We just celebrated with close family, went out for special meals and received gifts. However, our actual big parties were thrown in June. We could take our friends swimming, have bbq's, water balloon wars, all the things we couldn't do trapped up in the house. We looked forward to it every year. And, every single year, as adults and until my dad passed away, he sent us Un-Birthday cards.
My son's birthday is in late June, so we always have his parties before the school year lets out and people scatter to the winds for vacation. We always celebrate the actual day with family doing whatever he wants, something special, but for the actual party.......we do it when we know all his friends can attend. He's never felt robbed in any way by that. He gets to celebrate twice. It never seem weird for me and my sister or as if we were getting robbed. We had low-key winter celebrations and then a summer bust out! We had 6 months to plan our summer parties. I have always loved the heat of summer so getting my big party at that time of year never bothered me one bit.
I guess it depends on what works for different families. It worked for mine and as a kid, I loved it. So did my little sister.
It worked for us and we always looked forward to it.
Just my opinion.