We planted butternut squash that produced prolific fruit, but it just did not have flavor. The squash looked fine inside and out. What went wrong? Variety? Soil nutrients?
Unlike most garden vegetables where the fresher the better is the rule, butternut squash, like some other winter squash, actually improves with storage. (Don't Squander That Squash: Like Fine Wine it may Improve with Age)
The squash continues to develop after harvest, changing starches into sugars and increasing the carotenoids. They'll be at their best after a couple of months.
Harvest butternut squash when they are mature by cutting the stem about 2 inches from the fruit. Cure them to harden the skin at 80-85 degrees in the garden for 5-7 days. Store them in a dry dark space at about 50-55 degrees. This article has additional information, Winter Squash Eating Guide.