May 1st, also known as International Worker’s Day or May Day, is a commemoration of the 1886 Haymarket Massacre in Chicago while Chicago workers demonstrated on the street against working eight hours a day and the strike widely spread to many other cities such as New York and Baltimore. Although the strike was heavily suppressed, causing the deaths of hundreds people, it finally succeeded. Over 80 countries in the world officially regarded it as national holiday and many others celebrate it unofficially. In Vietnam, it is known as International Labor Day and considered as a public holiday.
As this event is adjacent to the Reunification Day, May Day is often jointed by April 30th to have a double-day off. In some years such as 2011 when both of these holidays fell on weekend, office can be closed for as long as 4 consecutive days.