Midday on Friday 16 December is the last day to order to get items before Christmas. Orders received that morning will go out by the end of that day. You can still order after this but due to how busy the post is you probably won't get it before the big day.
I send everything by first class post but even the Royal Mail have set the 16th as their first class pre-Christmas delivery cut-off. Usually it's a fair bit later than this but the industrial action has had a big impact on causing delays this year.
Most orders are being posted out in under a day of them arriving at the moment to try to give everything the best chance of getting to you and not getting delayed but delivery is always slower at this time of year.