The full moon of the 7th month is the Chinese holiday for the dead. This year, it is on August 8th. In Daoism, hell opens on the 1st day of this month (July 25th). All those lost spirits and ghosts roam out of hell to finish whatever they have to. This is also the month for them to feed, if they died hungry. Mortals everywhere offer fruits, meats, incenses, or whatever to appeace them. On the last day of this month (August 23rd), all of them go back to hell and the gates close.
The 7th month “leaps” this year. The 2nd 7th month starts on August 24th (new moon) until September 21st. I don't know whether the hell gets to stay open one extra month or not. Probably not.
During this month, it is not lucky to marry or made major purchases. Ghosts get jealous when mortals enjoy themselves too much and may ruin it with whatever ghosty tricks.
I found this tradition not much observed in mainland China. I am quite curious on why.