Well, of course the best time/day to go is the earliest day/time you possibly can .
If it will be anything like the last sample sale, there are lots of racks of DVF goodies organized by size. There are also boxes full of samples and stuff all jammed together. Oh, and don't arrive near closing time, because you won't have enough time to try everything on.
The dressing room--It's like you're a herd of cattle. You will have to fall in a very long line, where they admit 10-15 girls into the dressing room at one time. It's an open dressing room, so best not be shy! Wear a cami so you don't have to deal with your bra. There's no limit to how many clothes you can bring in (I think), but you only have 10 minutes to try on everything you bring in, so you have to be quick. Once the 10 minutes is up, you'll get kicked out.
My best tip would be to scope out your local DVF boutique and try on several different pieces for size, so you have a rough idea of what your DVF size is and how it will fit you, just in case you don't make it to the dressing room.
Hmm...even though I'm on a self-imposed shopping ban, thinking of this sale is really enticing me to go!!
During the after-work hours, there can be a line to enter the "store" and there will be a line for the dressing room.
The last few sales, I would get there around 4:30 and it wouldn't be too crowded, but by the time my friends showed up at 6, it was very crowded.
Prices usually get slashed closer to the end of the sale. I would generally go on the first day for must-haves and I always go on the last day for last-minute deals. This year, I'll be camping so my friends will be shopping for me.
I've been going to this sale for about five years and have always done well. There is something for pretty much every price point. I've gotten wrap dresses for $50 and others for $110.