As others have said, it depends on the market you're in and where you're going. There is no definitive answer to your question. Different parts of the country appear to have different patterns. On the west coast, for example, prices fluctuate constantly and the day of release is never the best price. In the Northeast, that pattern doesn't seem to be the case at all.
As for the day of the week of purchase, there again the is wide variability. In my market, Portland, OR, the day of the week matters a lot. For instance - just 2 weeks ago I was looking for airfare to MCO for a late October trip. I'd been watching fares for weeks and they hadn't moved off of $173 one way. When i looked on Saturday (the most expensive day to purchase) fares had gone up to $187. Checked again on Tuesday (typically the day of the week where fares are lowest here) and the fare was $113. I jumped on that. I looked again on Thursday and the fare went back up to $173. So they raised it over the weekend, and then lowered it briefly on Tuesday then went back to the "normal" fare. I've seen that pattern a lot here. Oh, and Holiday weekends are even worse times to purchase. Those same flights were $237 when I looked over the Labor Day weekend.
Its all driven by demand.
Steve