The Hungry Ghost festival may trigger the question, "Why are they called Hungry Ghosts? Are they in hunger?"
Well, according to this source of Wikipedia:
Hungry ghosts are not the same as ghosts in Chinese tradition. The traditional belief is that people become ghosts when they die, however, it was originally thought that ghosts did not have eternal life, but would slowly weaken and eventually die a second time. Hungry ghosts in traditional thought would only be an issue in exceptional cases, such as if a whole family were killed or when a family no longer venerated their ancestors. With the rise in popularity of Buddhism the idea that souls would live in space until reincarnation became popular. In the Taoist tradition it is believed that hungry ghosts can arise from people whose deaths have been violent or unhappy. Both Buddhism and Taoism share the idea that hungry ghosts can emerge from neglect or desertion of ancestors. According to the Hua-yen Sutra evil deeds will cause a soul to be reborn in one of six different realms. The highest degree of evil deed will cause a soul to be reborn as a denizen of hell, a lower degree of evil will cause a soul to be reborn as an animal, and the lowest degree will cause a soul to be reborn as a hungry ghost. According to the tradition, evil deeds that lead to becoming a hungry ghost are killing, stealing and sexual misconduct. Desire, greed, anger and ignorance are all factors in causing a soul to be reborn as a hungry ghost because they are motives for people to perform evil deeds.
The hungry ghosts are also classified as those whose families have forgotten about them or not tend to their graves. They are also believed to have thin long necks as they have not/never been fed by their families since their passing.
It is based on this notion that most practices and rituals during the Hungry Ghost Festival/Month to offer food and prayers to appease their spirits.
At the same time, the need to appease is to avoid any misfortune or haunting by the wandering souls among their household; or in short, unlikely mishaps.
Of course, despite being allowed to roam freely among the living during the whole month, my grandma also used to tell me that they have the guards from hell patrolling the Earth and watching over the ghosts to maintain security and peace among the living.
In fact, during the Yu Lan festival which falls on the 15th day of the month (or sometimes celebrated on the 14th in some places), you can see huge statues of the King of Hell/Great Guardian of Hell everywhere.
They are also like the law enforcers from hell to maintain discipline and to ensure that the spirits do not create chaos and will stick to their rules to go back to hell at the end of the month.
It is also a belief that we will observe rain throughout the month due to the yin energy and the dark clouds create the strong yin atmosphere to protect the spirits.
Heavy rain may be observed on the last day of the month due to the mourning and the reluctance of the spirits to go back or to bid farewell to their loved ones.
Well, I don't see any reason to doubt this as it has truly been raining for almost every day recently.