Check here: http://biz.yahoo.com/research/earncal/today.html Earnings announcements can come anytime. The day is usually known in advance, but revisions can come unexpectedly.
Its always been my observation that once I decided to go long on a position, guidance comes out contrary to my position. It works every time.
UG and DG tend to be pre and post market but there are always a few intraday UG/DG per day, and obviously given the market conditions of 2008 there have been a ton of playable intraday defences.
Equity research/sales teams try to get changes in recommendation out while the market is closed, so that the changes can be communicated, and clients can digest the research prior to the next open. In that window prior to the open, direct access to the analyst is often ~auctioned to valuable clients who express interest. Clients value, and pay for this type of service. Nowadays release of research is carefully managed by compliance departments that are tasked with ensuring equitable distribution to the firm's client base and other research outlets. Special circumstances sometimes prompt those same teams to make exceptions. For example, a company makes a surprise announcement near the open, and the stock makes an egregious move, the analyst may quickly issue a short note, changing his numbers and perhaps even his core view, so that the sales team can engage clients real-time. These types of mid-day upgrades/downgrades are usually swamped by reaction to the original public announcement however, and its generally a position an analyst does not like to be in. Ideally he is simply telling the sales force "I told you so" and referring them to prior research.
It seems like upgrades and downgrades usually happen before the open, but companies lowering guidance tend to do that after the close. And a study conducted a while back found that companies who report earnings after the close on a Friday are gonna have an earnings miss.
And when that downgrade happens before the open, there's no better feeling than being short the stock at Friday's close