Terrorism is ideologically motivated violence. The word has come to mean attacking ordinary people as complicitous in whatever offenses one is angry about, merely by their identity as enemy people. (This has been pointed out by the sociologist Jeff Goodwin.)
It attacks iconic targets and on iconic dates, because it is a war of emblems. It attacks places famous for gathering crowds, guaranteeing the presence of a lot of easy targets. Places of fun, celebration, partying, because these are most off guard. The Hamas attack on a psychedelic music festival near the Gaza border on October 7, 2023.
It is a war on fun: the serious and hate-filled versus the casual fun-lovers.
Hence the choice of places: Bourbon Street in the New Orleans French Quarter. Las Vegas-- sin city-- and for a hater of Trump and Musk, attacking their icons (hotels and cars) when their bodies are inaccessible. Attacking a Christmas market in Germany at a time when the government coalition has fallen and the Alt-Deutschland party has risen to its highest popularity.
The timing of such attacks on the same day need not be coincidental, nor a coordinated plot. Iconic dates attract iconic action.
In New York City in the 1950s, when people wanted to get together in Manhattan, it was conventional to say, meet me under the big clock in Grand Central Station. In those days of train travel, everyone knew the iconic waiting room with its big clock high on the walll.
Thomas Schelling, in his book The Strategy of Conflict, pointed out that strangers or enemies could arrive at a tacit agreement to coordinate, if there were some obvious choice. He had in mind a mutual halt in nuclear escalation, and gave the Grand Central clock as example of how tacit coordination works.
Suppose you had agreed to meet a friend in New York on a particular date, but you forgot to set a time and place. What do you do? If you are an experienced New Yorker, meet under the clock at noon-- the most obvious time of day if you have to guess what time the other person would guess.
For choosing a date of maximal public gathering, New Year's has similar iconic attraction. It is also a holiday, a time of leisure, peaking with the mindless cheering to greet the moment of climax on New Year's Eve. It is the universal night of celebration.
Ideological warriors are not necessarily in agreement with each other. But the attraction of iconic times and places makes them tacit allies. They are not even necessarily sane. There was a spate of threats, poisons, and hoaxes in the weeks after the 9/11/01 attacks. Roll them together and we get an accelerating tide of crises, some real, some hysterical.
"And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?" William Butler Yeats, 1921
Randall Collins. 2004. “Rituals of solidarity and security in the wake of terrorist attack.” Sociological Theory 22: 53-87.