Quote by kroberts11:
Largest gamerscore gain: 2/19/2008 (1,450) (I got a few easy sports games this day)
Largest weekly achievement gain: 1/4/2010 (43) (Dragon Age + Band Hero, I think)
Largest weekly gamerscore gain: 2/8/2010 (1,000) (I don't understand this, because my biggest gain in one day is 1450.. but whatever)
The thing about your largest weekly GS gain being smaller than your daily gain has to do with the fact that 360 Voice started keeping record of the weekly GS gain months after the daily one started and the staff explained that they couldn't go back and pull the old data from your daily record, even if it happened to be larger than the weekly record. Hopefully, this answers your question.
Back on topic, my achievements records:
Largest achievement gain: 6/29/2008 (28)
Largest gamerscore gain: 6/29/2008 (2,405) (Both this and the largest daily achievement gain came from Avatard, most of Madden 06 and some of Gun.)
Largest weekly achievement gain: 12/7/2009 (23)
Largest weekly gamerscore gain: 7/27/2009 (830)