The International Date Line (IDL) is an imaginary line of navigation on the surface of the Earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next. It passes through the middle of the Pacific Ocean, roughly following the 180° line of longitude but deviating to pass around some territories and island groups. 

The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian. In many places, the IDL follows the 180° meridian exactly. In other places, however, the IDL deviates east or west away from that meridian. These various deviations generally accommodate the political and/or economic affiliations of the affected areas. 

Proceeding from north to south, the first deviation of the IDL from 180° is to pass to the east of Wrangel Island and the Chukchi Peninsula, the easternmost part of Russian Siberia. (Wrangel Island lies directly on the meridian at 71°32′N 180°0′E, also noted as 71°32′N 180°0′W.) It then passes through the Bering Strait between the Diomede Islands at a distance of from each island at 168°58′37″ W. It then bends considerably west of 180°, passing west of St. Lawrence Island and St. Matthew Island.
what does it mark?
 change of one calendar day to the next