Retailers order a set number of each calendar and do not reorder. A box still in stock on Friday 27 November is, more often than not, one that did not sell — which is why the discounts land on the calendars nobody was fighting over. The premium beauty sets, the franchise collectibles and the single-malt runs tend to go in October and November at full price.
So the advice splits. If you have a particular calendar in mind, buy it when you see it; the risk you are carrying is that it disappears, not that it stays expensive. If you are flexible about which one you end up with, waiting costs you nothing, and the first half of December is when unsold stock gets genuinely cheap — a box with fewer days left in it than it has doors is a box a retailer wants gone.