You can drop the bag the previous evening, no problem. I am not sure if they will let you go through airside security the evening before (if from transit, yes, but not sure if you are from landside), but certainly the morning of your flight, if your aim is to go to the lounges. Note that the CX and QF lounges do not generally operate overnight. If you plan to take airport express to the airport, you can also check your bag at Hong Kong or Kowloon station, on the day or the day before.