Article II Section III gives the president the authority to call a special session of Congress. The DOJ apparently affirmed this authority in a 1947 opinion: https://www.justice.gov/file/20711/download

"The important factor would appear to be not the nature of the recess or ad- journment but, rather, that the Congress is not in session and that an extraordinary occasion has arisen which requires that it be in session and that it convene...

..., therefore, at a date earlier than it otherwise would. It is beyond question that the two houses of the Congress do not have the power, even by statute, to defeat the constitutional power of the President"