Announcement Date
  
Big news usually comes with a kind of notice or pre-announcing announcement, like "we will announce!" So what are they huzzah'ing about?
It could be a notification of the amount the next dividend will be and when it will take place. Or a company could announce the issue of a new bond and provide all the details to the public on the interest rate range, maturity date, interest payment dates, what the funds raised will be used for, and the offer price. Other official announcement dates are the all-important earnings reports, which tell how the company performed during the past quarter, often leading to the stock price going up or down depending on how good the results are vs. expectations.
A company might also announce a stock split or buy-back, or even a merger or acquisition. You want to be sure you are on the distribution list for any important announcement dates for companies you might want to invest in. If you are already a stock owner, you will be on the list.
If the company is public, on the heels of this announcement, they'll file an 8k form with the SEC, which makes sure pretty everyone can see it...it might even show up on the Batman projector thing in Gotham City.