UN English Day is observed annually on 23 April.
The event was established by UN's Department of Public Information in 2010 "to celebrate multilingualism and cultural diversity as well as to promote equal use of all six official languages throughout the Organization".
For the English Language Day, April 23 was chosen because it is the date "traditionally observed as both the birthday and date of death of William Shakespeare".
Other dates were selected for the celebration of the UN's other five official languages.
International Mother Language Day
Official languages of the United Nations
UN English Language Day Official Site
UN marks English Day as part of celebration of its six official languages, United Nations News Centre, 23 April 2010