Any body who put serious thought into it knew it was just a case of "They ran out of rock".
It fell exactly on a solstice .
Not having access to atomic clocks and such "primitive" societies went by lunar cycles, 13 months of 28 days with no leap anything.
Dec 21st is the last solstice of the year. It is unlikely that world apocalypse would fall on that particular day, just like it is unlikely to occur on Dec 31st.
But it was fun while it lasted and some of the idiots actually took it serious.
