The International Date Line is an imaginary line extending from the North Pole to the South Pole through the Pacific Ocean. It serves as the 180th meridian of longitude, and is used to designate the beginning of each calendar day. As you know, each adjacent time zone on the map has an hour time difference. However, at the International Date Line, +12 hours and -12 hours meet, bringing about a 24-hour time change. So while a person standing just to the west of the line may be celebrating Christmas Eve at 6 pm, someone just to the east will already be sitting down to Christmas dinner on December 25th. Therefore, when your ship crosses this line heading west, a day is added, and while crossing in an easterly direction, a day is subtracted.
Crossing the International Date Line has long been a rite of passage for sailors, who often must participate in a line-crossing ceremony to become part of the sacred "Order of the Golden Dragon", an honorary naval fraternity.