Ghost Month invented by Ming emperor, research shows 研究顯示：「鬼月」之說始於明太祖

Taipei Times

Thu, Aug 19, 2010 – Page 15

Buddhists associate the Ghost Festival with filial piety through the
legend of Maudgalyayana, who rescued his mother after she was reborn in
the realm of hungry ghosts. Yen says an expression used by the ancient
geomancers that, “Since ancient times, emperors have been buried in the
seventh month,” is further evidence that the seventh month used to be
considered auspicious. An alternative word for the seventh lunar month
contained the field radical with a vertical line through it, implying
that “The time is right in heaven, the conditions on earth are
favorable.” Yen says that Ming emperors not only had their burials in
this month, but also favored it for coronations.

So when did the seventh lunar month become inauspicious? Yen says
historical documents show that Liu Bo-wen, an advisor to the Hongwu
Emperor, proposed the idea of sending people disguised as Taoist priests
to cities and counties to spread the news that a disaster would happen
in the seventh month. From then on, the notion of that month being
unlucky began to spread.