July 25th, 2021
Also called Commonwealth Day or Occupation Day, this commemorates the proclamation of Puerto Rico's constitution in 1952. On July 3rd, 1952, the new constitution was approved by 82 percent of Puerto Ricans in a referendum. That same day, the governor signed it into law. However, the U.S. Congress proceeded to amend the new constitution on a few points and agreed to accept it only on condition of those changes being implemented. Thus, it took until July 25th for the new constitution to actually be put into effect.
Constitution Day will be celebrated on Monday, July 26 in 2021.
Source: www.publicholidays.la/puerto-rico/constitution-day/