It seems to reliably be the 24th (or a day or 2 before if the 24th is at a weekend). So I assume that it won't show in my bank account until Christmas Eve - a bit to late to buy Christmas prezzies but it will plug a few holes alright.
To be honest I didn't bank on anyone paying us before Christmas this year.. Last year we had to wait for payments to turn up by post so I made a mental note in my head we wouldn't get anymore money until Jan..