Hi, yes you can harvest it now, to test for storability, press with yout thumbnail if it leaves little or no indentation it will store fine, if it is soft it will store for less time. Unless it is very immature it should be fine for Halloween. Leave a length of stem on it on it, but don't pick it up by that. the length gives it less chance of getting bacteria into the fruit which might happen if you cut it close..oh and do cut it, don't pull it. Wipe it down with a mild solution of bleach, about 1-10 with water, this will kill any surface mold etc
If you put a candle in for Halloween. don't use for food afterwards.
Use any flesh you cut out for food when fresh and don't forget to roast the seeds when you take them out, they are good.
Have fun.
XX Jeannine