Opening Day

Opening Day is the day on which professional baseball leagues begin their regular season. For Major League Baseball and most of the minor leagues, this day typically falls during the first week of April. For baseball fans, Opening Day serves as a symbol of rebirth; writer Thomas Boswell once penned a book titled, Why Time Begins on Opening Day. Many feel that the occasion represents a newness or a chance to forget last season, in that all 30 of the major league clubs and their millions of fans begin with 0–0 records.

Opening Day festivities extend throughout the sport of baseball, to hundreds of Minor League Baseball franchises as well as to college, high school, youth league fields and in areas far beyond North America. Since Major League Baseball generally starts their season first among professional leagues, their Opening Day is the one most commonly recognized by the general public. Most of the minor leagues start a few days later, but within the same week; the short season Class A and Rookie leagues are exceptions, since they begin play in June. (College, high school and youth baseball seasons vary widely depending on location and weather conditions.) Opening Day ignores the exhibition games played during spring training in the month leading up to Opening Day.

Source: wikipedia.org

In 2017 Opening Day in USA falls on April 3.