Weekly Challenges are now available globally in Pokémon GO, giving players a fresh set of goals to chase every week. New challenges will drop every Monday at 4 p.m. PST, offering tasks that range from simple daily-style objectives to more involved activities that encourage exploring, battling and catching. The feature adds a steady rhythm to the game and gives players something consistent to look forward to without feeling overwhelming. It’s a solid addition for anyone who likes regular incentives and quick rewards, making each week feel a little more active and giving trainers another reason to log in and play.
Source: Official Pokémon GO blog