The best time to pick a yellow squash is when it is still relatively small. It is ready to be picked when it is about six inches long. If you let it grow much longer the skin will become thick and bitter. The best flavor will also come when it is about 6 inches long.
The squash will be a bright yellow color with few bruises or spots. It will be between 6 and 14 inches in length, firm and pull away from the vine and stems easily. It will also have a very faint scent and no holes from pests that have crawled inside.
Yellow squash will often produce small hard-like bumps along the entire outer skin, when it is ripe and ready to eat. It will also be firm, but not hard, and have a light hollow sound when you 'knock' on it.
Yellow Squash is ready to be picked when it's a solid yellow color and three to four inches long. I usually wait until it is about six to eight inches long, but it's a matter of preference. It'll be more tender the smaller and younger it is. As it gets older and longer it loses flavor and gets tougher.
Squash is determined to be picked by how long the actual squash is not by the blooming flowers like some people believe. You can pick it when it is only about 4 inches long and it will be nice and tender. Usually it is picked between five and seven inches long however.
Yellow squash is ready to be picked when there is no longer a yellow flower on the plant. The squash needs to be big enough to eat (at least 2 or 3 inches long). Actually, you can pick the squash at whatever size you would like it to be, but it is best when it comes off the vine.