The roots of the Office Xmas Party, you won't be surprised to learn, are pagan. It's a celebration of the Birthday of the Unconquered Sun (25 December, as it happened), of the old Yule, always observed most enthusiastically in these islands, and uninterrupted by the coming of Christianity.
The revels were a defiance of the darkest time of the year and a relief from the confines of the social order emphasised by the reversal of roles between ruler and ruled.
At mighty feasts in great halls, high Kings and fierce Thanes would serve their servants and serfs; for which read now MDs and CEOs, PAs, executives, and operatives, in tinselled conference rooms or anywhere with a house white. One of the lowlies was appointed Lord of Misrule to make things go with a swing; see now Doug from Accounts.
They might not have had stationery cupboards, but they had stamina, keeping at it for Twelve Days, something, I suspect, beyond even Alan from IT or Julie in Marketing.