So you've got 3 letters per day you can earn, with 3 letters getting you 1 Nice Point. This means the cap each week is essentially 7 Nice Points.
We're also going to have the ability to acquire Letters you missed in the Christmas Spirit Shop so you don't have to play every day to have equal opportunities. You earn Christmas Spirit by playing the seasonal activities at any time.
The countback on missed Letters is not week limited - so say if you missed week 1, you could technically earn up to 14 Nice Points that second week. You can't get more than the limit, but you can get as much as anyone else could have earned up to that point.
It's worth noting that you only need 1 Nice Point to get the new Christmas cosmetic from Santa each week - each Nice Point above that is just additional chances to roll the Black Party Hat (including just the 1 point giving you that chance as well). And if you miss any weeks, as long as you have 1 Nice Point, you'll get the Christmas cosmetics on offer from prior weeks as well.