Economic calendars are essential to any trader. They provide information about the upcoming economic events that might affect markets, events that good traders take into account when creating their trading strategies.
My free economic calendar of choice is provided by bloomberg, and can be found here.
This particular calendar shows a good level of detail on each event and accurate ideas of the markets predictions. Generally, if a report for example is predicted perfectly by the market, then the release of information should do very little to the financial product price.
The events vary in importance, there are some that I try to avoid when trading. If a GDP report for the U.S is due to be released the next day, and I have no predictions for it, then I will not be as inclined to short term invest in a stock as the report could have a dramatic influence on its price. Alternatively I could have a prediction for an event and invest accordingly.
So I urge you, if you start, or already are trading, please use your economic calendars!