I have heard squash left on the vine is sweeter after a frost, why?

In my opinion, this is practice developed for two reasons:

vegetables and fruits that are left on the vine longer, are able to become sweeter, because the acid decreases
the squash rind will become harder and more durable, and therefore the vegtable can last longer (if kept in the right conditions)

Although this method may have favorable outcomes, it is not necessary to wait until the first frost.

For more information on squash, visit: