A term of Congress begins on January 3 of odd years - following November elections which take place in even years. This is specified in the 20th Amendment of the Constitution.
The 20th Amendment allows Congress to change that date.
Congressional terms are numbered starting from the time the Constitution was enacted. The 114th Congress began on Jan 6, 2015, because the 113th Congress changed the date.
The party with with the most number of members (or which can create a majority by including independents) can elect a member of that party as Speaker of the House and Senate Majority Leader. It also can name members of that party as committee chairs.
In this chart, the colors (Blue = Democratic, Red = Republican) of the House Speaker and Senate Majority Leader indicate which party controlled that house during that term.
