If I were Greg Smith's publisher, I would have taken note that Goldman Sachs will be publishing their Q1 results on Tuesday 17th April and be making my publicity plans accordingly. I'd say that if Greg wants to maximise the sales of his book, he should be targetting this week for release.
If GS follow JP Morgan's rebound in business then the usual suspects will be bleating loudly about bankers' profits at the expense of the 99%. If instead they follow Citigroup's fall in profits then there's going to be a considerable amount of schadenfreude and "could this be the end?" speculation. Both of these are a great environment into which to release the book.