Here's how it works for me:
Place a present pile somewhere on the lot with space around it. It can be either the pile of boxes, or you can place a Christmas tree and then use the add present pile option on the tree to spawn the boxes.
At 8pm on Winterfest, Father Winter will arrive, do a belly laugh upon arrival and then if uninterrupted, he'll head for a present pile and fill it with enough gifts for the sims on the lot.
I haven't ever tried filling the present piles to capacity (I don't even know if they can have a max limit of presents?) But if you do, just make sure there's a present pile on the lot that isn't full. I have had it work in a three person household with a tree where I added three presents to the pile myself. Then when Father Winter arrived he seemed to add three more.
