https://www.gamespot.com/articles/destiny-2-nightfall-guide-the-arms-dealer-strikes-/1100-6454329/

Another weekly reset in Destiny 2 has occurred, bringing players a fresh slate of things to do, and moving the Flashpoint to Io. Most significantly, it rotates in the next Nightfall Strike, complete with new Modifiers and Challenges, as well as the opportunity to receive some nice rewards. [Update: It's Monday, making today your last chance to complete the Nightfall before the weekly reset.]