December — days, dates and countdowns

How to use December

December looks like 31 days and behaves like about 15 working ones in most places. The gap between those two numbers is where December plans go wrong. Before committing to anything in this month, count the days that are actually available rather than the days on the grid.

Mark the holidays for every country involved before scheduling anything. Assume the last two weeks have reduced availability, even where they are not formally holidays. Treat anything needing a reply from outside your team as having a hard cutoff around mid-month.

Year-end financial and reporting deadlines land on December 31 regardless of who is working.

The solstice and the turn

The December solstice falls in the second half of the month and marks the shortest day north of the equator and the longest south of it. After it, northern daylight begins lengthening again, though slowly enough that it is not noticeable for several weeks. The earliest sunset actually occurs slightly before the solstice and the latest sunrise slightly after — the two do not coincide, because the length of the solar day varies across the year.

This is why "the days are getting longer" and "it is still dark when I leave" can both be true in late December. The evening turn arrives before the morning one. December is the only month where "next year" and "the day after tomorrow" can mean the same thing, and where both sound equally like an excuse.

Why the twelfth is named tenth

From decem, Latin for ten. December closed the ten-month Roman year that began in March, and kept its name when January and February were added ahead of it. It is the last of the four mismatched months, and the only one whose original position was also the end of the year.

Year-end arithmetic

December is where date handling meets the year boundary, and the failures are consistent enough to list. Week numbering: the last days of December often belong to week 1 of the following year under ISO rules.

Adding a month to December 31 crosses into a new year and a shorter month at once. Fiscal years that do not end in December make "year to date" ambiguous without qualification. Any comparison of "this year versus last year" in late December compares an incomplete year to a complete one.

The last one is the most commonly missed. A December year-over-year chart that has not been adjusted for the incomplete final month will show a decline that is entirely an artefact of the calendar.

Closing the year properly

December 31 is a hard boundary for a lot of things — tax years in many countries, annual contracts, statistical reporting periods, calendar-year budgets. What makes it awkward is that the boundary is fixed while the working days around it are not. The deadline does not move because the office is closed.

The workable approach is to treat mid-December as the real deadline for anything requiring another person, and reserve the final stretch for work that can be done alone. Anything else depends on people being reachable at exactly the time of year when they most reliably are not.