December does not add work evenly. It compresses an unusual amount of it into three weeks that also contain school events, work parties, travel, and the assumption that everybody is relaxed.
The conflicts are knowable in November
Almost every December clash is visible weeks ahead: the school concert and the office party, the travel day and the delivery window, the two sets of relatives with incompatible expectations. Put the fixed dates down early and the negotiations happen while there is still room to negotiate.
Protect the empty days deliberately
The instinct is to fill the calendar because the invitations are pleasant. Block two or three days as unavailable before anything else lands on them. Slack in a schedule is not laziness — it is the only thing that absorbs the inevitable illness, delay, or thing that runs long.
And decide who owns what, once
Gifts, food, travel, cards, hosting. In most households these silently default to one person while everybody believes the load is shared. Naming an owner for each is a ten-minute conversation that changes the shape of the month.