I haven't asked her about that dress in particular. My guess is that since they just came out with two new colors, and the purple color is completely sold out, it won't go on sale until either right before, or after Christmas.
My personal shopper has been really perplexed with J. Crew's sales lately. It used to be more regular, but now they are marking things down randomly. Sometimes, something will be marked down in the store, that hasn't even shown up in the catalog yet or just came out in the catalog, so people see it and it's already gone or on sale.
My strategy is just to check once a week or so (when I remember). But now that they've come out with the lighter camel color (instead of the black I was planning), I'm much more tempted to buy it with my student discount than wait for it to go on sale......I don't know, I'll have to see how things pan out.