Title says it all
The reset day is Monday, so you’ll get the rewards from the reset day onwards, but not from the previous week.
If you’re looking for a new guild, mine is recruiting.
So when I join the new guild on Sunday, on Monday I’ll get the guild rewards even though I’ve only just joined?
Yes. All the guild statues reset after Sunday (i don’t know the exact time, but it’s around 8am GMT Monday morning for example).
You will get the rewards for guild tasks and legendaries completed starting after reset. You will not get any rewards that the guild earned the prior week, such as Guild Wars rewards.
It doesn’t matter how long you’ve been member, you just need to be around at the right time. More precisely, rewards are handed out to all members of a guild at the point in time the reward is earned. Guild war rewards are earned automatically on Monday at daily reset, task rewards are earned whenever a task is completed.