Hi! we will be in hongkong on sept.19-21. what will be the possible weather in those dates.

During that time, it will be about 25 C. And it will be just during the typhoon season, so the rain may happen from time to time. Therefore, you can take along some summer clothin, such as long-sleeved shirt and shorts, and a long-sleeved shirt can be needed in case of the rain and sudden temperature falling. Besides, the rainy wear is needed to be prepared.
Kindly suggestion for you: to pay attention to the weatehr forecast before outing may help a lot.