Here in the US the biggest shopping day of the Christmas season is the day after our Thanksgiving - which is always held on the last Thursday of Nov. - Nov. 28 this year. So the biggest day in the stores of brick and mortar at least - will be Nov. 29th.
In some ways it depends on what you are selling. One customer of ours sells gift items that are usually a last minute purchase so their biggest week is the week before Christmas. Some other sites see the few days after Thanksgiving as being the busiest.