Usually Acorn types of squash can be harvested whenever the skins become tough. Try sticking a fingernail into the skin and if it is difficult, then the squash is probably ready. Each variety will have a suggested maturity date and you can roughly judge the correct time by using that as a guideline. The squash will 'color up' to look like the mature version as well. Immature squash have a different appearance than the ones that are ready. The stem that attaches it to the vine will look a bit dried as well.