Flipside takes place 3 months before Burning Man, on Memorial Day weekend every year. That means May 22 to 25 this year, although I think people can stay through Monday to give further assistance with cleanup.
You can expect a wonderful time if you go. Burning Man veterans will find a familiar atmosphere in a far more intimate setting with a spirit that stands alone; those new to Burning Man events will find an event they will never forget in their lives. Having been to both events I can only urge you to discover the gem that is Flipside. You won't be sorry, and I want you there. You can read more about the event here.
Flipside is more than Burning Man with a creek, real toilets, a swimming pool, and hot showers, but if that is what it takes to get you out to Dripping Springs this year, then so be it.
(quicklinks: those tickets again, and my 2002 Flipside pictures 1, 2, and 3)