It isnt an official challenge, it is just an informal club where people make a post once a week to share their goals and progress update. You can see more info here:
@eddie-earner/saturday-savers-club-with-sally-saver-or-year-3-or-week-26-saturday-1-july-2023-or-win-eds-for-comments

@chaosmagic23 is also doing a weekly post.