are you using it today or do you want to store it?

today: doesn't matter. squashes are delicious immature! I always pick mine young, because you can get "old" ones year round from the grocer.

storage: Squash should be hard-shelled, doesn't give to pressure. twist vine gently to remove, the stem may still be attached, but not necessarily and indicator of full ripeness.
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I found this on allexperts.com:

"Wait until the green stripes become very faint... Squash, like tomatoes, will ripen off vine. But I never recommend this with either plant. The reason is you sacrifice flavor."

So, basically wait until the squash is no longer green. Other indications that the butternut squash is ripe is if it is over 8 inches in length, and if the green colored stem has faded brown, and has started to become brittle.
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