Harvest:
Harvest eggplants when they are firm and glossy and big enough to eat—about one-third of their maximum size. To test eggplant fruit for maturity, press the fruit with your thumb; if the flesh springs back, it’s green and not ripe; if your thumb leaves an indentation, the fruit is overripe; the best-tasting eggplant will be in between.

For more information on growing Eggplant, check out this great guide from Harvest To Table:
https://harvesttotable.com/harvest-store-eggplants/