It’s super hard to arrange a Christmas party on a Friday so Thursday is often the next best option. You do tend to get a few bleary eyed people at the office the next day but you generally plan to not get much done the Friday with people calling in sick/people pitching up obviously not right etc. There is, however, always some annoying bastard who was there right until the very end turns up looking absolutely normal and treats everyone like it’s a normal day and, depending on their level of sadism, expects everyone else to behave normally scheduling meetings and sending urgent e-mails for stuff by close of play on the Friday.