Daisy
Without knowing your timezone in relation to what Garmin may use as the timezone basis for the cutoffs for the challenges, it's difficult to say. Their help system says that it allows for 12 hours after the end of the challenge period but not in relation to what though. Perhaps doing as the person who posted prior to you and syncing your device Sunday evenings would work better.
Personally I am wondering if there is anything after the 125 000 steps / week challenge, if not I'll quit the challenges as I'm doing more of both running and walking and don't want dishearten others.