Halo Wars 2

Weekly Challenges are reset at 8pm PST, 11pm EST, or 7am GMT (depending on your time zone). Along with Daily Challenges, they can be accessed via the Main Menu by opening the "Challenges" tab.

Weekly Challenges are a little more time-consuming and difficult (not too difficult, though). A number of examples of a Weekly Challenge may be to complete twenty match-made games, complete nine matches as a specific leader, or obtain fifty counter-kills in a matchmaking. Upon completing a Weekly Challenge, re-opening the "Challenges" tab will show that specific one crossed off and replaced by a countdown timer as to when the next Challenge will become available.

All in all, you will need to complete all three Weekly Challenges in a single week in order to obtain this achievement.