If you are visiting a website, or reading a book, magazine, or other printed source, or viewing a plaque or sign in a museum or exhibit and you see the number 381, it is wrong.
The Montgomery Bus Boycott lasted 382 days, beginning on December 5, 1955, the first day of the protest, and ending on December 20, 1956, the day the U.S. Supreme Court order in the case of Browder v. Gayle was delivered to city and bus officials in Montgomery, Alabama. The Montgomery Improvement Association, the organization that led the boycott, then declared it over, and the next morning, December 21, 1956, Martin Luther King, Rosa Parks, Ralph Abernathy, Inez Baskin, and other black citizens boarded the buses and sat in whatever seats they chose. Thus ended segregated bus seating on municipal buses throughout the United States.
The reason for the 381 days error is that many journalists and even some historians did the math and counted the days between those two dates and came up with 381. But 1956 was a leap year, so February 29 was not taken into account in instances when the duration is given as 381 days. Add that Leap Day, and the duration of the boycott was 382 days.
The Montgomery Improvement Association noticed the error when it published in 1995 a commemorative 14-month calendar of the boycott, which showed that 382 was the correct number. The MIA has been trying to stamp out the error ever since.