In Article II, Section 3 of the Constitution, it states that the president has the power to call a Special Session. The reason why he rarely has to call a Special Session is because of the changes made by the 20th Amendment. Prior to the ratification of the 20th Amendment, Congress convened in December and typically adjourned in March. But after the ratification, Congress convenes on January 3 and typically does not adjourn until late in the year. So, the president rarely has to call Congress into session because Congress isn't generally out of session long.