Twitter Inc. /zigman2/quotes/203180645/composite TWTR +0.30% and Square Inc. /zigman2/quotes/205989440/composite SQ -0.90% will observe Juneteenth (June 19) as a company holiday in the U.S. -- the latest effort by tech leaders to honor the culture of blacks. "We will do the work to make those dates company holidays everywhere we are present," Jack Dorsey, the chief executive of both companies, tweeted Tuesday. Juneteenth is celebrated by many blacks as their emancipation from slavery in the U.S. It was on that date in 1865 that Union General Gordon Granger read the Emancipation Proclamation to African-Americans. Several tech companies -- including Apple Inc. /zigman2/quotes/202934861/composite AAPL +1.07% , Google parent Alphabet Inc. /zigman2/quotes/202490156/composite GOOGL +3.10% /zigman2/quotes/205453964/composite GOOG +2.90% , Intel Corp. /zigman2/quotes/203649727/composite INTC +4.13% , and Cisco Systems Inc. /zigman2/quotes/209509471/composite CSCO +3.79% -- have made pledges to nonprofit social-justice organizations and causes.