https://www.polygon.com/2018/10/2/17929806/destiny-2-forsaken-weekly-reset

Destiny 2: Forsaken is days away from being a month old, which means its ambitious three-week content cycle has already started over. The cycle in the game’s Dreaming City location didn’t start until the week after launch, and new content has been available each week since. But today’s reset brought with it an odd justification for why the reset is happening, and Bungie is leaning into it hard.

All games like Destiny have a weekly reset. To keep the game replayable, there has to be a day every week where your objectives start over again. With the Dreaming City, each week offers something new to do. As the recent Dreaming City curse grows greater, the content gets more difficult. This eventually culminates in the Shattered Throne dungeon every three weeks.