If it's the case of stregthening your team for playoffs, I find it the best time around ASW. Now the prices are starting to rise and they will keep rising until we reach playoffs.
Once the PO starts, and the first round ends, a massive dump of cheap players enter the market and they keep getting more and more expensive the closer the new season comes. Beginning of season - players expensive, but the prices fall within the first few weeks.
If we're talking about trainees - 1-2 weeks after the drafts. Most of the rich teams have already bought the strongest drafts and only the low to average spenders remain, with whom you can compete.