Summarize this article in one sentence.
The growing period for celery is much longer than 3 months, but they’re best when harvested immature. Wait for 3 months after transplanting, or 4 months after the seed was first started to harvest the stalks. The longer the celery grows, the tougher it will become. However, the tougher the celery, the more nutritious it tends to be. Harvest according to your personal preferences, but don’t wait longer than 5 months to harvest your first stalks! If you wait until the weather becomes too hot, the celery will be dry and bitter. Instead, harvest when temperatures are cool and the air is humid. If you like darker celery, wait until the temperature reaches around 70 °F (21 °C). The celery will be slightly tougher, but not inedible. Even the smallest stalks on the plant need to be at least 6 inches (15 cm) tall in order to be edible. The tallest stalks should be around 18 inches (46 cm), but can be much taller depending on the plant! If your celery plant hasn’t reached this size by the time you want to harvest it, wait another week and measure the stalks. To harvest a whole plant, you need to wait until it has reached its full size. Measure across the plant from one side to the other, through the center of the stalks. If the plant isn’t at least 3 inches (7.6 cm) wide, wait 1 to 2 weeks before harvesting it. It’s best to harvest whole plants that haven’t had any stalks removed. Check the top of the plant for open spaces between the stalks. They should be close together and moderately difficult to separate. If there are open spaces or the plant isn’t compact at the top, it’s best to harvest the individual stalks as they grow.

Summary:
Wait 3 to 5 months after planting the celery to harvest it. Harvest before the outdoor temperature reaches 65 °F (18 °C). Make sure the lower stalks are at least 6 in (15 cm) long. Ensure the plant is 3 in (7.6 cm) across to harvest all of the stalks. Look for stalks that are compact if you want to harvest the whole plant.