The fiscal year of most corporations follows the calendar year, so for the common first quarter, which runs January 1 through March 31, earnings season starts mid-April. Subsequent quarters close in June, September and December and their associated earnings seasons are in July, October and January. The length of earnings season can vary but generally it runs about six weeks.
Not all corporations report during earnings season, because some have fiscal years that don't run parallel to the calendar year. The Securities and Exchange Commission (SEC) allows a grace period before reporting is required. Furthermore, depending on corporate strategy and the relative success of the quarter, companies may be inclined to file their 10-Q and 10-K forms earlier or later.
The weeks leading up to earnings season are known as the quiet period. During the quiet period for any particular company, corporate insiders are prohibited from selectively divulging information to some investors before it's made publicly available. A similar restricted duration of time, the blackout period, prohibits insider trading during similar periods.