An offensive timed to the dark of the moon

The Tet Offensive opened during the lunar new year, which means it opened at new moon: about 0.2% of the Moon was lit, and the country was as dark as it gets.

Tet is the Vietnamese new year and, like every lunar new year, it is a new moon by definition — so a truce that everybody expected to be quiet was also the darkest night of the month. Some units attacked a day early, which is why the dates are given variously as the 30th and the 31st. The Moon is not a footnote here: a coordinated night assault on more than a hundred towns needs the dark, and the calendar handed it over on schedule.